Extensions: Add metrics for Web Request API actions.

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 4ae0fc1a0f6d2344445d30cee432445739e38446..1161b1ac9297aaa30d38cfc01467cb24b1b3bf32 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -19199,6 +19199,14 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Updates grouped by Extension::HistogramType.</summary>
 </histogram>
 
+<histogram name="Extensions.WebRequestAction" enum="WebRequestAction">
+  <owner>karandeepb@chromium.org</owner>
+  <summary>
+    Describes the action taken by the Web Request Extension API for a given
+    stage of a web request.

[Devlin, 2017/02/24 21:43:43]

We should mention when this is recorded, and how many times - e.g., I don't
think this is necessarily limited to only one recording per web request event,
right?

[karandeepb, 2017/02/25 03:55:19]

Oh yeah, I hadn't thought about it much. Now logging once for each distinct
"requested" action. 

+  </summary>
+</histogram>
+
 <histogram name="Extensions.WebRequestEventFoundFrame" units="boolean">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
@@ -111335,6 +111343,14 @@ value.
   <int value="2" label="WEB_HISTORY_QUERY_TIMED_OUT">Timed out</int>
 </enum>
 
+<enum name="WebRequestAction" type="int">
+  <int value="0" label="CANCEL"/>
+  <int value="1" label="REDIRECT"/>
+  <int value="2" label="MODIFY_REQUEST_HEADERS"/>
+  <int value="3" label="MODIFY_RESPONSE_HEADERS"/>
+  <int value="4" label="SET_AUTH_CREDENTIALS"/>
+</enum>
+
 <enum name="WebRtcAudioCodecs" type="int">
   <int value="0" label="Unknown"/>
   <int value="1" label="Opus"/>