[page_load_metrics] Add support for chrome:// urls and add extensions observer

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 "base/macros.h"
 #include "base/optional.h"
 #include "chrome/common/page_load_metrics/page_load_timing.h"
@@ skipping 106 matching lines @@
   // ResourceRequestInfo objects while this is the currently committed load in
   // the WebContents.
   int num_cache_requests;
   int num_network_requests;
 
   // Extra information supplied to the page load metrics system from the
   // renderer.
   const PageLoadMetadata metadata;
 };
 
+enum TrackingPolicy {
+  TRACK,
+  IGNORE,
+};
+
 // Interface for PageLoadMetrics observers. All instances of this class are
 // owned by the PageLoadTracker tracking a page load.
 class PageLoadMetricsObserver {
  public:
   virtual ~PageLoadMetricsObserver() {}
 
+  // Called before OnStart and OnCommit. If the method returns false, no further
+  // callbacks will occur in the observer.
+  virtual TrackingPolicy ShouldTrackLoad(

[Bryan McQuade, 2016/09/20 18:40:38]

i think a boolean is ok here - the method name is really clear about what a
boolean return value would indicate. do you think we need an enum?

[Charlie Harrison, 2016/09/20 22:46:23]

rdevlin recommended this on the doc. Fine to nix it though I don't feel strongly
either way.

+      content::NavigationHandle* navigation_handle);
+
   // The page load started, with the given navigation handle. Note that OnStart
   // is called for same-page navigations. Implementers of OnStart that only want
   // to process non-same-page navigations should also check to see that the page
   // load committed via OnCommit or committed_url in
   // PageLoadExtraInfo. currently_committed_url contains the URL of the
   // committed page load at the time the navigation for navigation_handle was
   // initiated, or the empty URL if there was no committed page load at the time
   // the navigation was initiated.
   virtual void OnStart(content::NavigationHandle* navigation_handle,
                        const GURL& currently_committed_url,
@@ skipping 86 matching lines @@
   // loads that result in downloads or 204s are aborted by the system, and are
   // also included as failed provisional loads.
   virtual void OnFailedProvisionalLoad(
       const FailedProvisionalLoadInfo& failed_provisional_load_info,
       const PageLoadExtraInfo& extra_info) {}
 };
 
 }  // namespace page_load_metrics
 
 #endif  // CHROME_BROWSER_PAGE_LOAD_METRICS_PAGE_LOAD_METRICS_OBSERVER_H_