Add ExternalPrerenderRequestHandler and related API

chrome/browser/prerender/prerender_origin.h

 // 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.
 
 #ifndef CHROME_BROWSER_PRERENDER_PRERENDER_ORIGIN_H_
 #define CHROME_BROWSER_PRERENDER_PRERENDER_ORIGIN_H_
 
 namespace prerender {
 
 // Origin indicates what caused the prerender.
 // NOTE: New values need to be appended, since they are used in histograms.
 enum Origin {
   // Obsolete: ORIGIN_LINK_REL_PRERENDER = 0,
   // Obsolete: ORIGIN_OMNIBOX_ORIGINAL = 1,
   ORIGIN_GWS_PRERENDER = 2,
   // Obsolete: ORIGIN_OMNIBOX_CONSERVATIVE = 3,
   // Obsolete: ORIGIN_OMNIBOX_EXACT = 4,
   ORIGIN_OMNIBOX = 5,
   ORIGIN_NONE = 6,
   ORIGIN_LINK_REL_PRERENDER_SAMEDOMAIN = 7,
   ORIGIN_LINK_REL_PRERENDER_CROSSDOMAIN = 8,
   ORIGIN_LOCAL_PREDICTOR = 9,
+  ORIGIN_EXTERNAL_REQUEST = 10,
   ORIGIN_MAX,
 };
 
 // Return a human-readable name for |origin|.
 const char* NameFromOrigin(Origin origin);
 
 }  // namespace prerender
 
 #endif  // CHROME_BROWSER_PRERENDER_PRERENDER_ORIGIN_H_