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..2142abb7c7bac6eb9306395e7228fa8167c52d1e 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -7919,6 +7919,15 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Extensions.BackgroundContentsServiceStartupTime"
+    units="milliseconds">
+  <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="Extensions.BackgroundPageLoadTime" units="milliseconds">
   <owner>kalman@chromium.org</owner>
   <summary>The time for an extension's background page to load.</summary>
@@ -8895,6 +8904,15 @@ 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>