Make three simultanious prerenders the default maximum in Canary and Dev only.

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 415b19bace613c8fb5ddd23a73e1a7e592a1af80..b6abfcaa6442ccb6a6d7b86bedda1936582032b1 100644
--- a/chrome/browser/prerender/prerender_histograms.cc
+++ b/chrome/browser/prerender/prerender_histograms.cc
@@ -148,7 +148,10 @@ void PrerenderHistograms::RecordPrerender(Origin origin, const GURL& url) {
   seen_pageload_started_after_prerender_ = false;
 }
 
-void PrerenderHistograms::RecordPrerenderStarted(Origin origin) const {
+void PrerenderHistograms::RecordPrerenderStarted(Origin origin,
+                                                 size_t prerender_count) const {
+  UMA_HISTOGRAM_ENUMERATION("Prerender.PrerenderCount", prerender_count,
+                            Config::kDefaultMaxConcurrency + 1);

[dominich, 2012/07/19 16:04:38]

this will be a measure of how often we prerender 1 vs 2 (vs ?) prererenders
concurrently? Does this number then need to be modulated by experiment, ie,
should we only record this for the experiment where maximum concurrency is > 1?
Or, even better, add a separate histogram for each maximum concurrency:

StringPrintf("Prerender.ConcurrentPrerenderCount%d",
PrerenderManager::config().concurrency)

[gavinp, 2012/07/23 18:35:21]

Good idea: I now only emit this if the max concurrency > 1. I also added the
integer to the name.

   if (OriginIsOmnibox(origin)) {
     UMA_HISTOGRAM_COUNTS(
         StringPrintf("Prerender.OmniboxPrerenderCount%s",