[Extensions] Add metrics for extension initialization

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 456e5f4a83e30f559265773946e25da076915bbf..0c5f4733e93c78aa252ed024eaba3feac9ae6575 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -16605,6 +16605,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Extensions.ExtensionCreationTime" units="microseconds">
+  <owner>rdevlin.cronin@chromium.org</owner>
+  <summary>
+    The amount of time taken to create a single extension object. This includes
+    parsing all the different extension manifest keys and initializating the
+    associated manifest data.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.ExtensionFrameMapCacheHit" units="Boolean">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
@@ -114074,6 +114083,13 @@ value.
   <affected-histogram name="Prerender.TimeUntilUsed2"/>
 </histogram_suffixes>
 
+<histogram_suffixes name="ProcessType" separator=".">
+  <suffix name="BrowserProcess" label="Browser Process"/>
+  <suffix name="RendererProcess" label="Renderer Process (non-extension)"/>

[Mark P, 2017/01/05 22:51:24]

Is there such thing as an "extension renderer process" different than a
"extension process"?  If not, I suggest the following change:
(non-extension)
->
(not an extension process)
or remove the parenthetical remark entirely.

[Devlin, 2017/01/10 23:17:16]

I went with the change -> (not an extension process).  I think the distinction
is important, since technically extension processes are all renderer processes.

+  <suffix name="ExtensionProcess" label="Extension Process"/>
+  <affected-histogram name="Extensions.ExtensionCreationTime"/>
+</histogram_suffixes>
+
 <histogram_suffixes name="ProfileMenuGAIASource" separator=".">
   <suffix name="NonGAIA" label="Interaction was not initiated from GAIA"/>
   <suffix name="GAIASignout"