Add UMA to determine how often we offline a page with previews.

chrome/browser/offline_pages/background_loader_offliner_unittest.cc

Index: chrome/browser/offline_pages/background_loader_offliner_unittest.cc
diff --git a/chrome/browser/offline_pages/background_loader_offliner_unittest.cc b/chrome/browser/offline_pages/background_loader_offliner_unittest.cc
index 2482234b4963e8f1be33df80e9115c5c5c6027e5..c97aa10484ccb0c43f4f7f338de183d972c9df03 100644
--- a/chrome/browser/offline_pages/background_loader_offliner_unittest.cc
+++ b/chrome/browser/offline_pages/background_loader_offliner_unittest.cc
@@ -11,6 +11,7 @@
 #include "base/test/scoped_mock_time_message_loop_task_runner.h"
 #include "base/threading/thread_task_runner_handle.h"
 #include "chrome/browser/android/offline_pages/offliner_helper.h"
+#include "chrome/browser/loader/chrome_navigation_data.h"
 #include "chrome/browser/net/prediction_options.h"
 #include "chrome/common/pref_names.h"
 #include "chrome/test/base/testing_profile.h"
@@ -570,6 +571,27 @@ TEST_F(BackgroundLoaderOfflinerTest, MAYBE_NoNextOnInternetDisconnected) {
   EXPECT_EQ(Offliner::RequestStatus::LOADING_FAILED_NO_NEXT, request_status());
 }
 
+TEST_F(BackgroundLoaderOfflinerTest, OffliningPreviewsStatusHistogram) {
+  base::Time creation_time = base::Time::Now();
+  SavePageRequest request(kRequestId, kHttpUrl, kClientId, creation_time,
+                          kUserRequested);
+  EXPECT_TRUE(offliner()->LoadAndSave(request, completion_callback(),
+                                      progress_callback()));
+
+  // Called after calling LoadAndSave so we have web_contents to work with.
+  std::unique_ptr<content::NavigationHandle> handle(
+      content::NavigationHandle::CreateNavigationHandleForTesting(
+          kHttpUrl, offliner()->web_contents()->GetMainFrame(), true,
+          net::Error::OK));
+  // Call DidFinishNavigation with handle.
+  offliner()->DidFinishNavigation(handle.get());
+
+  histograms().ExpectBucketCount(
+      "OfflinePages.Background.OffliningPreviewStatus.async_loading",
+      0,  // PreviewsOff

[RyanSturm, 2017/06/02 20:50:34]

Seems like you should have a test for PreviewsOn as well.

WebContentsTester exposes a method for setting the navigation data.

https://cs.chromium.org/chromium/src/content/public/test/web_contents_tester....

The implementation is fairly straightforward, so I believe you could just call
it with the web_contents_tester() and any navigationhandle.

[Pete Williamson, 2017/06/02 21:40:32]

I did try that, but it turns out to be problematic, because the call to
set_navigation_data() forced me to include a header file that is forbidden by
deps of the unit test.  Know any good workarounds?

[RyanSturm, 2017/06/02 22:14:25]

I think that header is already added to this unittest on line 31. Maybe you
tried to include content/browser/frame_host/navigation_handle_impl.h instead of
web_contents_tester.h

[Pete Williamson, 2017/06/02 23:17:41]

Yep, that worked, test added.  We now test both off and on.

+      1);
+}
+
 TEST_F(BackgroundLoaderOfflinerTest, OnlySavesOnceOnMultipleLoads) {
   base::Time creation_time = base::Time::Now();
   SavePageRequest request(kRequestId, kHttpUrl, kClientId, creation_time,