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 a79c63741b3999fb2ff859ef30221b3f6ad69300..657d0f674db3393e308d75990f3938009bdaa338 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"
@@ -156,6 +157,18 @@ void PrerenderHistograms::RecordPrerenderStarted(Origin origin) const {
   }
 }
 
+void PrerenderHistograms::RecordConcurrency(size_t prerender_count) const {
+  static const size_t kMaxRecordableConcurrency = 3;
+  const size_t max_concurrency = PrerenderManager::GetMaxConcurrency();
+  DCHECK_GE(kMaxRecordableConcurrency, max_concurrency);
+  if (max_concurrency > 1) {
+    UMA_HISTOGRAM_ENUMERATION(
+        StringPrintf("Prerender.PrerenderCountOf%" PRIuS "Max",
+                     max_concurrency),
+        prerender_count, kMaxRecordableConcurrency + 1);
+  }
+}
+
 void PrerenderHistograms::RecordUsedPrerender(Origin origin) const {
   if (OriginIsOmnibox(origin)) {
     UMA_HISTOGRAM_ENUMERATION(