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

chrome/common/page_load_metrics/page_track_decider.cc

Index: chrome/common/page_load_metrics/page_track_decider.cc
diff --git a/chrome/common/page_load_metrics/page_track_decider.cc b/chrome/common/page_load_metrics/page_track_decider.cc
index 457532a83f03eabfa76740f5d563d62c41859977..dd415c0c890a59c2fc1731beaadf1fc3c5471576 100644
--- a/chrome/common/page_load_metrics/page_track_decider.cc
+++ b/chrome/common/page_load_metrics/page_track_decider.cc
@@ -10,14 +10,13 @@ PageTrackDecider::PageTrackDecider() {}
 PageTrackDecider::~PageTrackDecider() {}
 
 bool PageTrackDecider::ShouldTrack() {
-  // Ignore non-HTTP schemes (e.g. chrome://).
-  if (!IsHttpOrHttpsUrl())
-    return false;
-
   // Ignore NTP loads.
   if (IsNewTabPageUrl())
     return false;
 
+  if (IsAboutBlankUrl())
+    return false;
+
   if (HasCommitted()) {
     // Ignore Chrome error pages (e.g. No Internet connection).
     if (IsChromeErrorPage())