Add UMA histogram timers for ProcessManager and BackgroundContentsService startup.

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 e38cc51e239ad3a23cda03f8503c7f5c2a14d980..80925dff85d155f039209bcf27c3fc9c469e386a 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1983,6 +1983,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="BackgroundContentsService.LoadOnExtensionsReadyTime">

[Ilya Sherman, 2015/02/05 02:50:26]

nit: Please add a units attribute.

[Ilya Sherman, 2015/02/05 02:50:26]

If this is really measuring something extensions-related, maybe it makes more
sense to group this under the Extensions group/namespace?  The dashboard is
geared toward having fewer groups with more histograms in each, rather than
having lots of small groups.

[Yoyo Zhou, 2015/02/05 03:08:02]

Done.

[Yoyo Zhou, 2015/02/05 03:08:02]

Done.

+  <owner>yoz@chromium.org</owner>
+  <summary>
+    Time taken to load BackgroundContents for apps at startup when the extension
+    system notifies that it is ready.
+  </summary>
+</histogram>
+
 <histogram name="BackgroundMode.OnStartup.AutoLaunchState"
     enum="AutoLaunchState">
   <owner>gab@chromium.org</owner>
@@ -8895,6 +8903,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Extensions.ProcessManagerStartupHostsTime"
+    units="milliseconds">
+  <owner>yoz@chromium.org</owner>
+  <summary>
+    The time taken to start up persistent background pages for extensions in
+    ExtensionProcessManager when the extension system notifies that it is ready.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.ResourceDirectoryTimestampQueryLatency"
     units="milliseconds">
   <owner>asargent@chromium.org</owner>