Add histograms to gather ResourceScheduler information.

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 514cf0cf09e654416b7c0f46af4c2d04f2b2d559..8f9f8a79c57d555bc11ab472905ce6214e243957 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -28071,6 +28071,22 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ResourceScheduler.ActiveRequestTimeDeferred">
+  <owner>aiolos@chromium.org</owner>
+  <summary>
+    The amount of time between when the ResourceScheduler sees a request and
+    when it removes it's throttle in a user observable client.

[Ilya Sherman, 2014/10/30 03:18:11]

nit: "user observable" -> "user-observable"

[Ilya Sherman, 2014/10/30 03:18:11]

nit: "it's" -> "its"

[mmenke, 2014/10/30 15:14:19]

"Removes its throttle" is a bit confusing - throttles live for the lifetime of a
request.  Maybe "stops throttling the request?"

[aiolos (Not reviewing), 2014/10/31 23:33:28]

Done.

[aiolos (Not reviewing), 2014/10/31 23:33:28]

Done.

+  </summary>
+</histogram>
+
+<histogram name="ResourceScheduler.BackgroundRequestTimeDeferred">
+  <owner>aiolos@chromium.org</owner>
+  <summary>
+    The amount of time between when the ResourceScheduler sees a request and
+    when it removes it's throttle in a background client.
+  </summary>
+</histogram>
+
 <histogram name="SafeBrowsing.EnabledSettingChanged" enum="BooleanEnabled">
   <owner>feng@chromium.org</owner>
   <summary>