Rework UMAHistogramHelper and StatisticsDeltaReader into [Chrome]HistogramTester.

chrome/browser/prerender/prerender_browsertest.cc

Index: chrome/browser/prerender/prerender_browsertest.cc
diff --git a/chrome/browser/prerender/prerender_browsertest.cc b/chrome/browser/prerender/prerender_browsertest.cc
index 20c9d5b6800ea097161ae01f275c3fdf1e1d924a..3f022ee08e0b1db2774e795174f27fc481f5a996 100644
--- a/chrome/browser/prerender/prerender_browsertest.cc
+++ b/chrome/browser/prerender/prerender_browsertest.cc
@@ -17,6 +17,7 @@
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/test/histogram_tester.h"
 #include "base/test/test_timeouts.h"
 #include "base/values.h"
 #include "chrome/browser/browsing_data/browsing_data_helper.h"
@@ -64,7 +65,6 @@
 #include "chrome/test/base/in_process_browser_test.h"
 #include "chrome/test/base/test_switches.h"
 #include "chrome/test/base/ui_test_utils.h"
-#include "chrome/test/base/uma_histogram_helper.h"
 #include "content/public/browser/browser_message_filter.h"
 #include "content/public/browser/devtools_agent_host.h"
 #include "content/public/browser/devtools_client_host.h"
@@ -1507,6 +1507,10 @@ class PrerenderBrowserTest : virtual public InProcessBrowserTest {
                            js).c_str()));
   }
 
+  base::HistogramTester* const histogram_tester() {

[Ilya Sherman, 2014/07/16 18:44:46]

You're currently returning a const pointer, in the sense that the pointer value
is const.  That's not very helpful -- the pointer value is a primitive type, and
so would be copied anyway.  What you want is "const base::HistogramTester*" --
equivalently, but less idiomatically, "base::HistogramTester const * -- which
specifies that the pointed-to object is const.

Of course at that point, since the object is never null, you might as well use a
const-reference: "const base::HistogramTester&".

[Mike Lerman, 2014/07/16 19:36:36]

I'd say I hate C++ const, except Java's final isn't much better.

+    return &histograms_;
+  }
+
  protected:
   bool autostart_test_server_;
 
@@ -1657,21 +1661,22 @@ class PrerenderBrowserTest : virtual public InProcessBrowserTest {
   std::string loader_path_;
   std::string loader_query_;
   Browser* explicitly_set_browser_;
+  base::HistogramTester histograms_;
 };
 
 // Checks that a page is correctly prerendered in the case of a
 // <link rel=prerender> tag and then loaded into a tab in response to a
 // navigation.
 IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderPage) {
-  UMAHistogramHelper histograms;
-
   PrerenderTestURL("files/prerender/prerender_page.html", FINAL_STATUS_USED, 1);
   EXPECT_EQ(1, GetPrerenderDomContentLoadedEventCountForLinkNumber(0));
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatched", 0);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatchedComplete", 0);
-  histograms.ExpectTotalCount("Prerender.websame_PrerenderNotSwappedInPLT", 1);
+  histogram_tester()->ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatched", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatchedComplete", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.websame_PrerenderNotSwappedInPLT", 1);
 
   ChannelDestructionWatcher channel_close_watcher;
   channel_close_watcher.WatchChannel(
@@ -1679,10 +1684,10 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderPage) {
   NavigateToDestURL();
   channel_close_watcher.WaitForChannelClose();
 
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.websame_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.websame_PerceivedPLTMatched", 1);
-  histograms.ExpectTotalCount(
+  histogram_tester()->ExpectTotalCount("Prerender.websame_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.websame_PerceivedPLTMatched", 1);
+  histogram_tester()->ExpectTotalCount(
       "Prerender.websame_PerceivedPLTMatchedComplete", 1);
 
   ASSERT_TRUE(IsEmptyPrerenderLinkManager());
@@ -1690,21 +1695,21 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderPage) {
 
 // Checks that cross-domain prerenders emit the correct histograms.
 IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderPageCrossDomain) {
-  UMAHistogramHelper histograms;
-
   PrerenderTestURL(GetCrossDomainTestUrl("files/prerender/prerender_page.html"),
                    FINAL_STATUS_USED, 1);
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatched", 0);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatchedComplete", 0);
-  histograms.ExpectTotalCount("Prerender.webcross_PrerenderNotSwappedInPLT", 1);
+  histogram_tester()->ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatched", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatchedComplete", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.webcross_PrerenderNotSwappedInPLT", 1);
 
   NavigateToDestURL();
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.webcross_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.webcross_PerceivedPLTMatched", 1);
-  histograms.ExpectTotalCount(
+  histogram_tester()->ExpectTotalCount("Prerender.webcross_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.webcross_PerceivedPLTMatched", 1);
+  histogram_tester()->ExpectTotalCount(
       "Prerender.webcross_PerceivedPLTMatchedComplete", 1);
 }
 
@@ -2083,7 +2088,6 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest,
 IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest,
                        PrerenderLocationReplaceGWSHistograms) {
   DisableJavascriptCalls();
-  UMAHistogramHelper histograms;
 
   // The loader page should look like Google.
   const std::string kGoogleDotCom("www.google.com");
@@ -2109,14 +2113,16 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest,
   EXPECT_TRUE(DidPrerenderPass(prerender->contents()->prerender_contents()));
   EXPECT_EQ(1, prerender->number_of_loads());
 
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatched", 0);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatchedComplete", 0);
+  histogram_tester()->ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatched", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatchedComplete", 0);
   // Although there is a client redirect, it is dropped from histograms because
   // it is a Google URL. The target page itself does not load until after the
   // swap.
-  histograms.ExpectTotalCount("Prerender.gws_PrerenderNotSwappedInPLT", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.gws_PrerenderNotSwappedInPLT", 0);
 
   GURL navigate_url = test_server()->GetURL(
       "files/prerender/prerender_location_replace.html?" +
@@ -2134,15 +2140,17 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest,
 
   EXPECT_TRUE(DidDisplayPass(GetActiveWebContents()));
 
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.gws_PrerenderNotSwappedInPLT", 0);
-  histograms.ExpectTotalCount("Prerender.gws_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.gws_PerceivedPLTMatched", 1);
-  histograms.ExpectTotalCount(
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.gws_PrerenderNotSwappedInPLT", 0);
+  histogram_tester()->ExpectTotalCount("Prerender.gws_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.gws_PerceivedPLTMatched", 1);
+  histogram_tester()->ExpectTotalCount(
       "Prerender.gws_PerceivedPLTMatchedComplete", 1);
 
   // The client redirect does /not/ count as a miss because it's a Google URL.
-  histograms.ExpectTotalCount("Prerender.PerceivedPLTFirstAfterMiss", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.PerceivedPLTFirstAfterMiss", 0);
 }
 
 // Checks that client-issued redirects work with prerendering.
@@ -3348,24 +3356,25 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, ControlGroupRendererInitiated) {
 // account for the MatchComplete case, and it must have a final status of
 // FINAL_STATUS_WOULD_HAVE_BEEN_USED.
 IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, MatchCompleteDummy) {
-  UMAHistogramHelper histograms;
 
   std::vector<FinalStatus> expected_final_status_queue;
   expected_final_status_queue.push_back(FINAL_STATUS_INVALID_HTTP_METHOD);
   expected_final_status_queue.push_back(FINAL_STATUS_WOULD_HAVE_BEEN_USED);
   PrerenderTestURL("files/prerender/prerender_xhr_put.html",
                    expected_final_status_queue, 1);
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatched", 0);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatchedComplete", 0);
-  histograms.ExpectTotalCount("Prerender.websame_PrerenderNotSwappedInPLT", 1);
+  histogram_tester()->ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatched", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatchedComplete", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.websame_PrerenderNotSwappedInPLT", 1);
 
   NavigateToDestURL();
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.websame_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.websame_PerceivedPLTMatched", 0);
-  histograms.ExpectTotalCount(
+  histogram_tester()->ExpectTotalCount("Prerender.websame_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.websame_PerceivedPLTMatched", 0);
+  histogram_tester()->ExpectTotalCount(
       "Prerender.websame_PerceivedPLTMatchedComplete", 1);
 }
 
@@ -3373,7 +3382,6 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, MatchCompleteDummy) {
 // progress does not also classify the previous navigation as a MatchComplete.
 IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest,
                        MatchCompleteDummyCancelNavigation) {
-  UMAHistogramHelper histograms;
 
   // Arrange for a URL to hang.
   const GURL kNoCommitUrl("http://never-respond.example.com");
@@ -3391,11 +3399,13 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest,
   expected_final_status_queue.push_back(FINAL_STATUS_WOULD_HAVE_BEEN_USED);
   PrerenderTestURL("files/prerender/prerender_xhr_put.html",
                    expected_final_status_queue, 1);
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatched", 0);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatchedComplete", 0);
-  histograms.ExpectTotalCount("Prerender.websame_PrerenderNotSwappedInPLT", 1);
+  histogram_tester()->ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatched", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatchedComplete", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.websame_PrerenderNotSwappedInPLT", 1);
 
   // Open the hanging URL in a new tab. Wait for both the new tab to open and
   // the hanging request to be scheduled.
@@ -3408,13 +3418,15 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest,
   // should forcibly complete the previous navigation and also complete a
   // WOULD_HAVE_BEEN_PRERENDERED navigation.
   NavigateToDestURL();
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLT", 2);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatched", 0);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatchedComplete", 0);
-  histograms.ExpectTotalCount("Prerender.websame_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.websame_PerceivedPLTMatched", 0);
-  histograms.ExpectTotalCount(
+  histogram_tester()->ExpectTotalCount("Prerender.none_PerceivedPLT", 2);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatched", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatchedComplete", 0);
+  histogram_tester()->ExpectTotalCount("Prerender.websame_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.websame_PerceivedPLTMatched", 0);
+  histogram_tester()->ExpectTotalCount(
       "Prerender.websame_PerceivedPLTMatchedComplete", 1);
 }
 
@@ -3797,7 +3809,6 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, CancelMatchCompleteDummy) {
 // visible. Also test the right histogram events are emitted in this case.
 IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderDeferredImage) {
   DisableJavascriptCalls();
-  UMAHistogramHelper histograms;
 
   // The prerender will not completely load until after the swap, so wait for a
   // title change before calling DidPrerenderPass.
@@ -3809,11 +3820,13 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderDeferredImage) {
   EXPECT_EQ(1, GetPrerenderDomContentLoadedEventCountForLinkNumber(0));
   EXPECT_TRUE(DidPrerenderPass(prerender->contents()->prerender_contents()));
   EXPECT_EQ(0, prerender->number_of_loads());
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatched", 0);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatchedComplete", 0);
-  histograms.ExpectTotalCount("Prerender.websame_PrerenderNotSwappedInPLT", 0);
+  histogram_tester()->ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatched", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatchedComplete", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.websame_PrerenderNotSwappedInPLT", 0);
 
   // Swap.
   NavigationOrSwapObserver swap_observer(current_browser()->tab_strip_model(),
@@ -3829,11 +3842,13 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderDeferredImage) {
   // Now check DidDisplayPass.
   EXPECT_TRUE(DidDisplayPass(GetActiveWebContents()));
 
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.websame_PrerenderNotSwappedInPLT", 0);
-  histograms.ExpectTotalCount("Prerender.websame_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.websame_PerceivedPLTMatched", 1);
-  histograms.ExpectTotalCount(
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.websame_PrerenderNotSwappedInPLT", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.websame_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.websame_PerceivedPLTMatched", 1);
+  histogram_tester()->ExpectTotalCount(
       "Prerender.websame_PerceivedPLTMatchedComplete", 1);
 }
 
@@ -4178,14 +4193,14 @@ IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderPing) {
 }
 
 IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest, PrerenderPPLTNormalNavigation) {
-  UMAHistogramHelper histograms;
 
   GURL url = test_server()->GetURL("files/prerender/prerender_page.html");
   ui_test_utils::NavigateToURL(current_browser(), url);
-  histograms.Fetch();
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatched", 0);
-  histograms.ExpectTotalCount("Prerender.none_PerceivedPLTMatchedComplete", 0);
+  histogram_tester()->ExpectTotalCount("Prerender.none_PerceivedPLT", 1);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatched", 0);
+  histogram_tester()->ExpectTotalCount(
+      "Prerender.none_PerceivedPLTMatchedComplete", 0);
 }
 
 IN_PROC_BROWSER_TEST_F(PrerenderBrowserTest,