Change multi-prerender API to include per launcher slots.

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 7395842983b118ce103b84a153af916f7041ffe3..dc13c794222bb40776172e593376c06c3f4a158b 100644
--- a/chrome/browser/prerender/prerender_histograms.cc
+++ b/chrome/browser/prerender/prerender_histograms.cc
@@ -169,12 +169,12 @@ void PrerenderHistograms::RecordPrerenderStarted(Origin origin) const {
 
 void PrerenderHistograms::RecordConcurrency(size_t prerender_count,
                                             size_t max_concurrency) const {

[mmenke, 2012/12/18 17:15:15]

Since there is no longer a real "max_concurrency", I think it's a little odd to
take it as an argument.

[gavinp, 2012/12/18 20:15:12]

But max_link_concurrency  is also used in here as a proxy for being on the multi
prerender experiment. So I've moved a test on that experiment into prerender
manager.

-  static const size_t kMaxRecordableConcurrency = 3;
+  static const size_t kMaxRecordableConcurrency = 20;
   DCHECK_GE(kMaxRecordableConcurrency, max_concurrency);
   if (max_concurrency > 1) {
     UMA_HISTOGRAM_ENUMERATION(
         StringPrintf("Prerender.PrerenderCountOf%" PRIuS "Max",
-                     max_concurrency),
+                     kMaxRecordableConcurrency),
         prerender_count, kMaxRecordableConcurrency + 1);
   }
 }