Add metrics for schemes of content setting exceptions

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 7dd4a3842746ee713a6c2e134eb3d2656a10f118..fdbeb55a25dccb7fd3ab54818bcaeb219d63e4c7 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -6555,6 +6555,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="ContentSettings.ExceptionScheme" enum="ContentSettingScheme">
+  <owner>lshang@chromium.org</owner>
+  <summary>
+    The number of different content setting exception schemes at browser start.

[Ilya Sherman, 2016/08/10 05:34:26]

If I understand this description correctly, then I think this means something
like "Records the content setting exception schemes present at browser start.  
If multiple exceptions with a single scheme exist, the corresponding bucket will
be incremented multiple times."

If I'm understanding correctly, please use that description, or a
modified/even-clearer version of it.  If I'm misunderstanding, please correct
the description as needed.

[lshang, 2016/08/11 00:26:50]

Done. Yeah you understand correctly, I've changed the summary to be more clear.

+  </summary>
+</histogram>
+
 <histogram name="ContentSettings.LastSettingParsed" enum="LastSettingParsed">
   <obsolete>
     Deprecated 2015-10-05 in Issue 433475. Histogram was used temorarily for
@@ -70663,6 +70670,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <int value="3" label="Clicked 'Learn more'"/>
 </enum>
 
+<enum name="ContentSettingScheme" type="int">
+  <int value="0" label="(wildcard)"/>
+  <int value="1" label="http"/>
+  <int value="2" label="https"/>
+  <int value="3" label="file"/>
+  <int value="4" label="chrome-extension"/>
+  <int value="5" label="(other)"/>

[Ilya Sherman, 2016/08/10 05:34:26]

I'd recommend listing unknown/other as the first bucket.  That way, if you ever
add any additional known schemes, the "other" bucket won't end up somewhere in
the middle of the list.

[lshang, 2016/08/11 00:26:50]

Done. Moved all known schmes down to the list.

+</enum>
+
 <enum name="ContentType" type="int">
   <int value="-1" label="Invalid setting"/>
   <int value="0" label="Cookies setting"/>