Add ExternalPrerenderRequestHandler and related API

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 7e7fd42836e0f29aaa611ff96d5e8a748ccc4469..d1dcf959139f38ea428b935f59741b0290462af0 100644
--- a/chrome/browser/prerender/prerender_histograms.cc
+++ b/chrome/browser/prerender/prerender_histograms.cc
@@ -58,6 +58,8 @@ std::string GetHistogramName(Origin origin, uint8 experiment_id,
       return ComposeHistogramName("webcross", name);
     case ORIGIN_LOCAL_PREDICTOR:
       return ComposeHistogramName("localpredictor", name);
+    case ORIGIN_EXTERNAL_REQUEST:
+        return ComposeHistogramName("externalrequest", name);
     case ORIGIN_GWS_PRERENDER:  // Handled above.
     default:
       NOTREACHED();
@@ -117,6 +119,8 @@ bool OriginIsOmnibox(Origin origin) {
     HISTOGRAM; \
   } else if (origin == ORIGIN_LOCAL_PREDICTOR) { \
     HISTOGRAM; \
+  } else if (origin == ORIGIN_EXTERNAL_REQUEST) { \
+    HISTOGRAM; \
   } else if (experiment != kNoExperiment) { \
     HISTOGRAM; \
   } else { \