Add histogram to track prerender sessions

chrome/browser/browser_main.cc

Index: chrome/browser/browser_main.cc
diff --git a/chrome/browser/browser_main.cc b/chrome/browser/browser_main.cc
index 38e9d337971a565056a5dcc3ee3f5f28a554ff1c..a64a6906b2502eba6a59d0a4fde0d296df5df2c1 100644
--- a/chrome/browser/browser_main.cc
+++ b/chrome/browser/browser_main.cc
@@ -1581,6 +1581,10 @@ int BrowserMain(const MainFunctionParams& parameters) {
   RecordBreakpadStatusUMA(metrics);
   about_flags::RecordUMAStatistics(user_prefs);
 
+  UMA_HISTOGRAM_ENUMERATION(

[cbentzel, 2011/01/23 12:32:47]

Could you put this in PrefetchFieldTrial so it's more tied with the other
prerender/prefetch?

This is OK as well since near other UMA calls. 

+      "Prerender.Sessions",
+      parsed_command_line.HasSwitch(switches::kEnablePagePrerender), 2);

[cbentzel, 2011/01/23 12:32:47]

Eventually I think this histogram will want more buckets such as off, manually
disabled, manually enabled, trial-enabled.

[jar (doing other things), 2011/01/23 20:15:53]

I think HasSwitch returns a boolean, but the HISTOGRAM is expected an integer
sample.  I'd like to see a more explicit selection of 0 vs 1.

[gavinp, 2011/01/24 06:01:46]

OK, I've now created an enumeration (which I put in the prerendering directory),
and I use that to seed the histogram.  That should also allow expansion of the
types in this histogram for different session types.  Chris suggested we'd want
this if we don't use the UMA field trial mechanism, and there might well be
other reasons.

+
   // Stat the directory with the inspector's files so that we can know if we
   // should display the entry in the context menu or not.
   browser_process->CheckForInspectorFiles();