Add ExternalPrerenderRequestHandler and related API

chrome/browser/prerender/prerender_origin.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "chrome/browser/prerender/prerender_origin.h"
 
 #include "base/metrics/histogram.h"
 #include "chrome/browser/prerender/prerender_manager.h"
 
 namespace prerender {
 
 namespace {
 
 const char* kOriginNames[] = {
   "[Deprecated] Link Rel Prerender (original)",
   "[Deprecated] Omnibox (original)",
   "GWS Prerender",
   "[Deprecated] Omnibox (conservative)",
   "[Deprecated] Omnibox (exact)",
   "Omnibox",
   "None",
   "Link Rel Prerender (same domain)",
   "Link Rel Prerender (cross domain)",
   "Local Predictor",
+  "External Request",
   "Max",
 };
 COMPILE_ASSERT(arraysize(kOriginNames) == ORIGIN_MAX + 1,
                PrerenderOrigin_name_count_mismatch);
 
 }  // namespace
 
 const char* NameFromOrigin(Origin origin) {
   DCHECK(static_cast<int>(origin) >= 0 &&
          origin <= ORIGIN_MAX);
   return kOriginNames[origin];
 }
 
 }  // namespace prerender