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

chrome/common/page_load_metrics/page_track_decider.cc

 // 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.
 
 #include "chrome/common/page_load_metrics/page_track_decider.h"
 
 namespace page_load_metrics {
 
 PageTrackDecider::PageTrackDecider() {}
 PageTrackDecider::~PageTrackDecider() {}
 
 bool PageTrackDecider::ShouldTrack() {
-  // Ignore non-HTTP schemes (e.g. chrome://).
-  if (!IsHttpOrHttpsUrl())
+  // Ignore NTP loads.
+  if (IsNewTabPageUrl())
     return false;
 
-  // Ignore NTP loads.
-  if (IsNewTabPageUrl())
+  if (IsAboutBlankUrl())
     return false;
 
   if (HasCommitted()) {
     // Ignore Chrome error pages (e.g. No Internet connection).
     if (IsChromeErrorPage())
       return false;
 
     // Ignore non-HTML documents (e.g. SVG, images).
     if (!IsHtmlOrXhtmlPage())
       return false;
 
     // Ignore network error pages (e.g. 4xx, 5xx).
     int http_status_code = GetHttpStatusCode();
     if (http_status_code > 0 &&
         (http_status_code < 200 || http_status_code >= 400))
       return false;
   }
 
   return true;
 }
 
 }  // namespace page_load_metrics