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

chrome/common/page_load_metrics/page_track_decider.h

Index: chrome/common/page_load_metrics/page_track_decider.h
diff --git a/chrome/common/page_load_metrics/page_track_decider.h b/chrome/common/page_load_metrics/page_track_decider.h
index a656fffc5fa34d538c5cf362d1c818ab416f3de4..6b7658c38a9e951120c1ce2f76cf71c8bd571fcf 100644
--- a/chrome/common/page_load_metrics/page_track_decider.h
+++ b/chrome/common/page_load_metrics/page_track_decider.h
@@ -25,15 +25,12 @@ class PageTrackDecider {
   // Whether the navigation for the current page has committed.
   virtual bool HasCommitted() = 0;
 
-  // Whether the current page's URL is HTTP or HTTPS. Note that the page does
-  // not need to have committed, nor does the URL or hostname of the URL need to
-  // point at a valid web page or host, for this method to return true.
-  virtual bool IsHttpOrHttpsUrl() = 0;
-
   // Whether the current page's URL is for a Chrome new tab page. Note that in
   // some cases the new tab page is served over the network via https.
   virtual bool IsNewTabPageUrl() = 0;
 
+  virtual bool IsAboutBlankUrl() = 0;
+
   // The methods below can only be called if HasCommitted() returns true.
 
   // Whether the current page is a Chrome-generated error page. Example error