[NoStatePrefetch] Add UMA histogram to count prefetch requests

chrome/browser/prerender/prerender_histograms.cc

Index: chrome/browser/prerender/prerender_histograms.cc
diff --git a/chrome/browser/prerender/prerender_histograms.cc b/chrome/browser/prerender/prerender_histograms.cc
index 3e4b4bfa1915016af5bb4cf47d24037264b2f2f0..5b17d6bcbb10fc6dbb3f4b3cc28cbe44d2822ebc 100644
--- a/chrome/browser/prerender/prerender_histograms.cc
+++ b/chrome/browser/prerender/prerender_histograms.cc
@@ -19,6 +19,19 @@ namespace prerender {
 
 namespace {
 
+// This enum is used to define the buckets for the
+// "Prerender.NoStatePrefetchResourceCount" histogram family.
+// Hence, existing enumerated constants should never be deleted or reordered,
+// and new constants should only be appended at the end of the enumeration.
+enum NoStatePrefetchResourceType {
+  kMainResourceCacheable = 0,

[pasko, 2016/08/30 13:41:04]

I think chromium style prefers MACRO_STYLE constants, at least it is so in
chrome/browser/prerender/, let's keep it this way for consistency.

Also, c++11 promotes using 'scoped enums' aka 'enum class Foo {...};'

[droger, 2016/08/30 14:31:41]

changed the naming style, however I need these to be convertible to int, so I
didn't use the enum class.

+  kMainResourceNoStore = 1,
+  kSubResourceCacheable = 2,
+  kSubResourceNoStore = 3,
+
+  kNoStatePrefetchResourceTypeCount  // Must be the last.
+};
+
 // Time window for which we will record windowed PLTs from the last observed
 // link rel=prefetch tag. This is not intended to be the same as the prerender
 // ttl, it's just intended to be a window during which a prerender has likely
@@ -383,6 +396,27 @@ void PrerenderHistograms::RecordNetworkBytes(Origin origin,
   }
 }
 
+void PrerenderHistograms::RecordResourcePrefetch(Origin origin,
+                                                 bool is_main_resource,
+                                                 bool is_no_store) {
+  NoStatePrefetchResourceType type =
+      is_main_resource
+          ? (is_no_store ? kMainResourceNoStore : kMainResourceCacheable)
+          : (is_no_store ? kSubResourceNoStore : kSubResourceCacheable);
+  DCHECK_LT(type, kNoStatePrefetchResourceTypeCount);
+
+  std::string histogram_name =
+      GetHistogramName(origin, IsOriginWash(), "NoStatePrefetchResourceCount");
+
+  // Unrolls UMA_HISTOGRAM_ENUMERATION, required to support dynamic histogram

[pasko, 2016/08/30 13:41:04]

Would it be possible to DCHECK_CURRENTLY_ON(BrowserThread::UI) here? AFAIR
FactoryGet is not thread safe.

[droger, 2016/08/30 14:31:41]

Done (using a thread checker).

+  // name.
+  base::HistogramBase* histogram_pointer = base::LinearHistogram::FactoryGet(
+      histogram_name, 1, kNoStatePrefetchResourceTypeCount,
+      kNoStatePrefetchResourceTypeCount + 1,
+      base::HistogramBase::kUmaTargetedHistogramFlag);
+  histogram_pointer->Add(type);
+}
+
 bool PrerenderHistograms::IsOriginWash() const {
   if (!WithinWindow())
     return false;