Adding UseCounter specific for extensions

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 4f1ec4167292e4484b030fa076457f8e5f1b56f0..fa5614d53dbd607f66002cd1b32d4df9d06498f3 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -5281,6 +5281,16 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </details>
 </histogram>
 
+<histogram name="Blink.UseCounter.Extensions.Features" enum="FeatureObserver">
+  <owner>rbyers@chromium.org</owner>
+  <summary>
+    Count of how many page loads use various features for extension pages only.

[Rick Byers, 2017/04/28 15:04:40]

nit: Say explicitly "pages with a chrome-extension://" URL to eliminate any
possible ambiguity around what an "extension page" is.

[lunalu1, 2017/05/01 15:37:52]

Done

+    The PageVisits bucket is incremented for each page load, and the other
+    buckets incremented at most once per PageVisit via the WebCore::UserCounter
+    class.
+  </summary>
+</histogram>
+
 <histogram name="Blink.UseCounter.Features" enum="FeatureObserver">
   <owner>rbyers@chromium.org</owner>
   <summary>