Update prerender policy for custom tabs

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 b7401d0a9dfb948579de7e529bd3b33be1e19ace..6e46198b70abc42630c98e6a9b5bfa2a5f0a2118 100644
--- a/chrome/browser/prerender/prerender_histograms.cc
+++ b/chrome/browser/prerender/prerender_histograms.cc
@@ -54,6 +54,8 @@ std::string GetHistogramName(Origin origin, bool is_wash,
       return ComposeHistogramName("webnext", name);
     case ORIGIN_GWS_PRERENDER:
       return ComposeHistogramName("gws", name);
+    case ORIGIN_EXTERNAL_REQUEST_FORCED_CELLULAR:
+      return ComposeHistogramName("externalrequestforced", name);
     default:
       NOTREACHED();
       break;
@@ -105,6 +107,8 @@ do { \
     HISTOGRAM; \
   } else if (origin == ORIGIN_LINK_REL_NEXT) { \
     HISTOGRAM; \
+  } else if (origin == ORIGIN_EXTERNAL_REQUEST_FORCED_CELLULAR) { \
+    HISTOGRAM; \
   } else { \
     HISTOGRAM; \
   } \