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

chrome/browser/page_load_metrics/page_load_metrics_observer.h

Index: chrome/browser/page_load_metrics/page_load_metrics_observer.h
diff --git a/chrome/browser/page_load_metrics/page_load_metrics_observer.h b/chrome/browser/page_load_metrics/page_load_metrics_observer.h
index 9262a358aa96cea00330c86a143f83aa89708c66..9620c2b236c6340192ba1c4173e7f25a2b0eb76e 100644
--- a/chrome/browser/page_load_metrics/page_load_metrics_observer.h
+++ b/chrome/browser/page_load_metrics/page_load_metrics_observer.h
@@ -124,12 +124,22 @@ struct PageLoadExtraInfo {
   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