Add UMA histogram for suppressed dialogs on prerender swap.

Get statistics on how often OnCloseWebContentsDeleter would have showed a
dialog, presumably for onbeforeunload, just to see how often this actually
occurs.

BUG=304932

Review URL: https://codereview.chromium.org/58053002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@232908 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/prerender/prerender_manager.cc

@@ -160,7 +160,8 @@ class PrerenderManager::OnCloseWebContentsDeleter
   OnCloseWebContentsDeleter(PrerenderManager* manager,
                             WebContents* tab)
       : manager_(manager),
-        tab_(tab) {
+        tab_(tab),
+        suppressed_dialog_(false) {
     tab_->SetDelegate(this);
     base::MessageLoop::current()->PostDelayedTask(FROM_HERE,
         base::Bind(&OnCloseWebContentsDeleter::ScheduleWebContentsForDeletion,
@@ -179,6 +180,9 @@ class PrerenderManager::OnCloseWebContentsDeleter
   }
 
   virtual bool ShouldSuppressDialogs() OVERRIDE {
+    // Use this as a proxy for getting statistics on how often we fail to honor
+    // the beforeunload event.
+    suppressed_dialog_ = true;
     return true;
   }
 
@@ -186,13 +190,17 @@ class PrerenderManager::OnCloseWebContentsDeleter
   static const int kDeleteWithExtremePrejudiceSeconds = 3;
 
   void ScheduleWebContentsForDeletion(bool timeout) {
+    UMA_HISTOGRAM_BOOLEAN("Prerender.TabContentsDeleterTimeout", timeout);
+    UMA_HISTOGRAM_BOOLEAN("Prerender.TabContentsDeleterSuppressedDialog",
+                          suppressed_dialog_);
     tab_->SetDelegate(NULL);
     manager_->ScheduleDeleteOldWebContents(tab_.release(), this);
-    UMA_HISTOGRAM_BOOLEAN("Prerender.TabContentsDeleterTimeout", timeout);
+    // |this| is deleted at this point.
   }
 
   PrerenderManager* manager_;
   scoped_ptr<WebContents> tab_;
+  bool suppressed_dialog_;
 
   DISALLOW_COPY_AND_ASSIGN(OnCloseWebContentsDeleter);
 };
@@ -584,6 +592,7 @@ bool PrerenderManager::MaybeUsePrerenderedPage(const GURL& url,
 
   if (old_web_contents->NeedToFireBeforeUnload()) {
     // Schedule the delete to occur after the tab has run its unload handlers.
+    // TODO(davidben): Honor the beforeunload event. http://crbug.com/304932
     on_close_web_contents_deleters_.push_back(
         new OnCloseWebContentsDeleter(this, old_web_contents));
     old_web_contents->GetRenderViewHost()->

tools/metrics/histograms/histograms.xml

@@ -13880,7 +13880,16 @@ other types of suffix sets.
   </summary>
 </histogram>
 
-<histogram name="Prerender.TabContentsDeleterTimeout">
+<histogram name="Prerender.TabContentsDeleterSuppressedDialog"
+    enum="BooleanSuppressed">
+  <summary>
+    A boolean that indicates how often we suppress a dialog from a tab when
+    swapping it with a prerender.
+  </summary>
+</histogram>
+
+<histogram name="Prerender.TabContentsDeleterTimeout"
+    enum="BooleanCloseTimeout">
   <summary>
     A boolean that indicates how often we fail to delete an old prerendered tab
     before the timeout.
@@ -20378,6 +20387,11 @@ other types of suffix sets.
   <int value="1" label="Attempted"/>
 </enum>
 
+<enum name="BooleanCloseTimeout" type="int">
+  <int value="0" label="Closed normally"/>
+  <int value="1" label="Timed out"/>
+</enum>
+
 <enum name="BooleanCommonNameMatch" type="int">
   <int value="0" label="subjectAltName used"/>
   <int value="1" label="Common Name used"/>
@@ -20473,6 +20487,11 @@ other types of suffix sets.
   <int value="1" label="Success"/>
 </enum>
 
+<enum name="BooleanSuppressed" type="int">
+  <int value="0" label="No suppressions"/>
+  <int value="1" label="Suppressed"/>
+</enum>
+
 <enum name="BooleanTabDiscard" type="int">
   <int value="0" label="Memory OK, no discards"/>
   <int value="1" label="Memory low, tabs discarded"/>