[Extensions] Add metrics for UserScriptListener

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 0d361e4473fa557ce783a67fdf6f31d6eafc83ac..13bc3aca3b200bfdaf573cc1e7952b12de27c196 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -9854,6 +9854,24 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="Extensions.ThrottledNetworkRequestDelay" units="milliseconds">
+  <owner>rdevlin.cronin@chromium.org</owner>
+  <owner>kalman@chromium.org</owner>
+  <summary>
+    The duration for which a network request was delayed while waiting for
+    extension user script load.

[Mark P, 2015/03/19 18:59:15]

Is this recorded on every network request?  (What if there are no extension user
scripts to load?)  Or on every network request where there is an extension user
script that needs to load?  What happens if multiple extension user scripts need
to load--do you record this once or each time?

similar questions apply to the below histogram

[Devlin, 2015/03/19 19:45:54]

Tried to clear it up a bit - sound better?

[Mark P, 2015/03/19 20:19:01]

Definitely better, thanks.

+  </summary>
+</histogram>
+
+<histogram name="Extensions.ThrottledNetworkRequests" units="requests">
+  <owner>rdevlin.cronin@chromium.org</owner>
+  <owner>kalman@chromium.org</owner>
+  <summary>
+    The total number of network requests (per profile) that were throttled by

[Mark P, 2015/03/19 18:59:15]

What does "per profile" mean here?  How could this not be "per profile"?  Either
you're implying that the same network request could be delayed on separate
profiles (which makes no sense because profiles shouldn't share network
requests) or the same extension user script load can delay network requests
across multiple profiles (which also makes no sense because extensions are
per-profile).

[Devlin, 2015/03/19 19:45:54]

Cleared up.  But could potentially be not-per-profile if we emitted this once
per browser session, and tracked the total number of network requests that were
delayed across all profiles.

[Mark P, 2015/03/19 20:19:01]

Nice job clearing it up.

+    extension user script load.

[Mark P, 2015/03/19 18:59:15]

When is this emitted?  Periodically?

By the way, does throttled = delayed?  You use "delayed" in the other histogram
description.

[Devlin, 2015/03/19 19:45:54]

Emitted once per profile when all user scripts have loaded (added to the
description).  s/throttle/delay.

+  </summary>
+</histogram>
+
 <histogram name="Extensions.ToolstripLoadTime" units="milliseconds">
   <owner>Please list the metric's owners. Add more owner tags as needed.</owner>
   <summary>Time taken to load a toolstrip.</summary>