Add support for counting same-document AMP loads.

chrome/browser/page_load_metrics/page_load_metrics_observer.h

 // Copyright 2015 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_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_
 #define CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_
 
 #include <memory>
 #include <string>
 
@@ skipping 286 matching lines @@
   // Observers that return STOP_OBSERVING will not receive any additional
   // callbacks, and will be deleted after invocation of this method returns.
   virtual ObservePolicy OnCommit(content::NavigationHandle* navigation_handle);
 
   // OnDidFinishSubFrameNavigation is triggered when a sub-frame of the
   // committed page has finished navigating. It has either committed, aborted,
   // was a same document navigation, or has been replaced. It is up to the
   // observer to query |navigation_handle| to determine which happened. Note
   // that |navigation_handle| will be destroyed soon after this call. Don't
   // hold a reference to it.
-  virtual ObservePolicy OnDidFinishSubFrameNavigation(
-      content::NavigationHandle* navigation_handle);
+  virtual void OnDidFinishSubFrameNavigation(
+      content::NavigationHandle* navigation_handle) {}
+
+  // OnCommitSameDocumentNavigation is triggered when a same-document navigation
+  // commits within the main frame of the current page. Note that
+  // |navigation_handle| will be destroyed soon after this call. Don't hold a
+  // reference to it.
+  virtual void OnCommitSameDocumentNavigation(
+      content::NavigationHandle* navigation_handle) {}
 
   // OnHidden is triggered when a page leaves the foreground. It does not fire
   // when a foreground page is permanently closed; for that, listen to
   // OnComplete instead.
   virtual ObservePolicy OnHidden(const PageLoadTiming& timing,
                                  const PageLoadExtraInfo& extra_info);
 
   // OnShown is triggered when a page is brought to the foreground. It does not
   // fire when the page first loads; for that, listen for OnStart instead.
   virtual ObservePolicy OnShown();
@@ skipping 118 matching lines @@
   // Called whenever a request is loaded for this page load. This comes
   // unfiltered from the ResourceDispatcherHost and may include blob requests
   // and data uris.
   virtual void OnLoadedResource(
       const ExtraRequestCompleteInfo& extra_request_complete_info) {}
 };
 
 }  // namespace page_load_metrics
 
 #endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_