Add instrumentation to see how often a local predictor prerender could be

chrome/browser/prerender/prerender_local_predictor.cc

Index: chrome/browser/prerender/prerender_local_predictor.cc
===================================================================
--- chrome/browser/prerender/prerender_local_predictor.cc	(revision 199567)
+++ chrome/browser/prerender/prerender_local_predictor.cc	(working copy)
@@ -306,7 +306,8 @@
     PrerenderManager* prerender_manager)
     : prerender_manager_(prerender_manager),
       is_visit_database_observer_(false),
-      weak_factory_(this) {
+      weak_factory_(this),
+      current_prerender_would_have_matched_(false) {
   RecordEvent(EVENT_CONSTRUCTED);
   if (MessageLoop::current()) {
     timer_.Start(FROM_HERE,
@@ -731,6 +732,7 @@
       prerender_handle_.release());
   prerender_handle_.reset(prerender_manager_->AddPrerenderFromLocalPredictor(
       url, session_storage_namespace.get(), *size));
+  current_prerender_would_have_matched_ = false;
   if (old_prerender_handle)
     old_prerender_handle->OnCancel();
 
@@ -757,4 +759,21 @@
   current_prerender_->actual_start_time = current_time;
 }
 
+void PrerenderLocalPredictor::OnTabHelperURLSeen(
+    const GURL& url, WebContents* web_contents) {
+  RecordEvent(EVENT_TAB_HELPER_URL_SEEN);

[Shishir, 2013/05/14 18:19:34]

This event will have a lot of duplicates since it is called from both
provisional load start and commit. Ideally you would want cleaner stats.

[tburkard, 2013/05/14 19:28:06]

We already have other events for when a DidNavigate happens, so we can compare
against these if we want cleaner stats.

On 2013/05/14 18:19:34, Shishir wrote:

+  if (current_prerender_would_have_matched_ ||
+      !prerender_handle_.get() ||
+      !prerender_handle_->Matches(url, NULL)) {

[Shishir, 2013/05/14 18:19:34]

Can you call this with NULL if the prerender is live? There is a check in
prerender contents that would likely fail.

[tburkard, 2013/05/14 19:28:06]

Done.

+    return;
+  }
+  RecordEvent(EVENT_TAB_HELPER_URL_SEEN_MATCH);
+  if (prerender_handle_->Matches(
+          url,
+          web_contents->GetController().GetDefaultSessionStorageNamespace())) {
+    RecordEvent(EVENT_TAB_HELPER_URL_SEEN_NAMESPACE_MATCH);
+  }
+  current_prerender_would_have_matched_ = true;

[Shishir, 2013/05/14 18:19:34]

Can you add a comment as to the conditions where this can happen. Ideally if the
URL and namespace matches, this would have gotten swapped in.

[tburkard, 2013/05/14 19:28:06]

Done.

+}
+
 }  // namespace prerender