Refactor Extension initialization logic. Add tracing and additional histogram.

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index 1ed23b8d7698329a5e7688892b3e8f1f7d4675e0..fec533680ca9a0a002f2ff0bfcb396ca32ba3d67 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -8329,6 +8329,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Extensions.CheckForExternalUpdatesTime">
+  <owner>rkaplow@chromium.org</owner>
+  <summary>
+    The amount of time that elapsed during
+    ExtensionService::CheckForExternalUpdates.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.CorruptExtensionBecameDisabled">
   <owner>asargent@chromium.org</owner>
   <summary>
@@ -8923,6 +8931,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Extensions.LoadAllTime2" units="milliseconds">
+  <owner>kalman@chromium.org</owner>
+  <summary>
+    Time taken to load all non-component extensions at browser startup and
+    record metrics. This happens as part of the total initialization time of
+    ExtensionService, measured in Extensions.ExtensionServiceInitTime.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.LoadApp">
   <owner>kalman@chromium.org</owner>
   <summary>The number of apps loaded by each user at startup time.</summary>