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..6dc7b306ae22059329e96c7096f455891d202cee 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -28071,6 +28071,56 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   </summary>
 </histogram>
 
+<histogram name="ResourceScheduler.RequestTimeDeferred_Active">
+  <owner>aiolos@chromium.org</owner>
+  <summary>
+    The amount of time the ResourceScheduler is throttling a request after
+    WillStartRequest is called in a user-observable client.
+  </summary>
+</histogram>
+
+<histogram name="ResourceScheduler.RequestTimeDeferred_Background">
+  <owner>aiolos@chromium.org</owner>
+  <summary>
+    The amount of time the ResourceScheduler is throttling a request after
+    WillStartRequest is called in a background client.
+  </summary>
+</histogram>
+
+<histogram name="ResourceScheduler.RequestTimeDeferred_Other">
+  <owner>aiolos@chromium.org</owner>
+  <summary>
+    The amount of time the ResourceScheduler is throttling a request after
+    WillStartRequest is called on a request without a Client or a request in a
+    Client which has switched states since the request was made.
+  </summary>
+</histogram>
+
+<histogram name="ResourceScheduler.RequestTimeThrottled_Active">
+  <owner>aiolos@chromium.org</owner>
+  <summary>
+    The amount of time between when the request was created and when the
+    ResourceScheduler stops throttling the request in a user-observable client.
+  </summary>
+</histogram>
+
+<histogram name="ResourceScheduler.RequestTimeThrottled_Background">
+  <owner>aiolos@chromium.org</owner>
+  <summary>
+    The amount of time between when the request was created and when the
+    ResourceScheduler stops throttling the request in a background client.
+  </summary>
+</histogram>
+
+<histogram name="ResourceScheduler.RequestTimeThrottled_Other">
+  <owner>aiolos@chromium.org</owner>
+  <summary>
+    The amount of time between when the request was created and when the
+    ResourceScheduler stops throttling a request in a Client which has switched
+    states since the request was made or a request without a Client.

[mmenke, 2014/11/03 19:01:20]

optional:  Worth mentioning this isn't perfectly accurate, here or in the code? 
If a tab goes background -> active -> background while a resource loads, it will
be in the Background histogram.  I think this is probably not worth fixing, but
is worth noting somewhere.

[aiolos (Not reviewing), 2014/11/03 20:16:02]

I think it would be worthwhile putting it in both places, but agree that we
don't care about fixing.
Done.

+  </summary>
+</histogram>
+
 <histogram name="SafeBrowsing.EnabledSettingChanged" enum="BooleanEnabled">
   <owner>feng@chromium.org</owner>
   <summary>