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

chrome/browser/prerender/prerender_manager.cc

Index: chrome/browser/prerender/prerender_manager.cc
diff --git a/chrome/browser/prerender/prerender_manager.cc b/chrome/browser/prerender/prerender_manager.cc
index b43eec6bd94e869a3cf20555003360251379897b..ab818ae8473c6728ede53048e8575a8ce842dda6 100644
--- a/chrome/browser/prerender/prerender_manager.cc
+++ b/chrome/browser/prerender/prerender_manager.cc
@@ -69,9 +69,6 @@ const int kWindowDurationSeconds = 30;
 // Time interval at which periodic cleanups are performed.
 const int kPeriodicCleanupIntervalMs = 1000;
 
-// Time interval before a new prerender is allowed.
-const int kMinTimeBetweenPrerendersMs = 500;
-
 // Valid HTTP methods for prerendering.
 const char* const kValidHttpMethods[] = {
   "GET",
@@ -196,6 +193,7 @@ PrerenderManager::PrerenderManager(Profile* profile,
   // Any other checks simply make sure that the PrerenderManager is accessed on
   // the same thread that it was created on.
   DCHECK(BrowserThread::CurrentlyOn(BrowserThread::UI));
+  config_.max_concurrency = GetMaxConcurrency();
 }
 
 PrerenderManager::~PrerenderManager() {
@@ -555,6 +553,8 @@ const char* PrerenderManager::GetModeString() {
       return "_Enabled";
     case PRERENDER_MODE_EXPERIMENT_CONTROL_GROUP:
       return "_Control";
+    case PRERENDER_MODE_EXPERIMENT_MULTI_PRERENDER_GROUP:
+      return "_Multi";
     case PRERENDER_MODE_EXPERIMENT_5MIN_TTL_GROUP:
       return "_5MinTTL";
     case PRERENDER_MODE_EXPERIMENT_NO_USE_GROUP:
@@ -586,6 +586,13 @@ bool PrerenderManager::IsNoUseGroup() {
   return GetMode() == PRERENDER_MODE_EXPERIMENT_NO_USE_GROUP;
 }
 
+// static
+size_t PrerenderManager::GetMaxConcurrency() {
+  if (GetMode() == PRERENDER_MODE_EXPERIMENT_MULTI_PRERENDER_GROUP)
+    return 3;
+  return 1;
+}
+
 bool PrerenderManager::IsWebContentsPrerendering(
     WebContents* web_contents) const {
   DCHECK(CalledOnValidThread());
@@ -945,12 +952,14 @@ PrerenderHandle* PrerenderManager::AddPrerender(
                                         session_storage_namespace,
                                         control_group_behavior);
 
-  while (active_prerender_list_.size() > config_.max_elements) {
+  while (active_prerender_list_.size() > config_.max_concurrency) {
     prerender_contents = active_prerender_list_.front()->contents_;
     DCHECK(prerender_contents);
     prerender_contents->Destroy(FINAL_STATUS_EVICTED);
   }
 
+  histograms_->RecordConcurrency(active_prerender_list_.size());
+
   StartSchedulingPeriodicCleanups();
   return prerender_handle;
 }