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..734d89b0deca3f5f914089fcdde2edc3952427af 100644
--- a/chrome/browser/prerender/prerender_histograms.cc
+++ b/chrome/browser/prerender/prerender_histograms.cc
@@ -6,6 +6,7 @@
 
 #include <string>
 
+#include "base/format_macros.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
 #include "base/stringprintf.h"
@@ -157,6 +158,17 @@ void PrerenderHistograms::RecordPrerenderStarted(Origin origin) const {
   }
 }
 
+void PrerenderHistograms::RecordConcurrency(size_t prerender_count) const {
+  const size_t max_concurrency = PrerenderManager::GetMaxConcurrency();
+  if (max_concurrency > 1) {
+    UMA_HISTOGRAM_ENUMERATION(
+        StringPrintf("Prerender.PrerenderCountOf%" PRIuS "Max",
+                     max_concurrency),
+        prerender_count, Config::kMaximumMaxConcurrency + 1);
+  }
+}
+
+
 void PrerenderHistograms::RecordUsedPrerender(Origin origin) const {
   if (OriginIsOmnibox(origin)) {
     UMA_HISTOGRAM_COUNTS(