Add canvas context type histogram

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 fecd8a04c4665a07428dc687595e45e57840bd57..fc048fa1729fd53db345724aab62c663f1a422da 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -1085,6 +1085,13 @@ other types of suffix sets.
   </summary>
 </histogram>
 
+<histogram name="Canvas.ContextType" enum="CanvasContextType">
+  <summary>
+    The context type passed into the getContext() canvas function. Used to track

[Mark P, 2013/08/02 14:46:14]

This sounds icky.  Most histograms are emitted on a clearly defined basis: on
startup, on profile load, on navigation, when the user does X, when this network
event occurs, etc.  It's not obvious to me what causes a function like
getContext() to be called.  I can easily imagine another programmer to add
another call to a function named getContext() rather than, say, pass the context
down through six layers of function parameters.  I can imagine if you have
multiple calls in close proximity that some may be optimized away b by the
compiler.

Can you please make the histogram emit at a more definitive time?  (or argue all
my worries are inappropriate)

[Sami, 2013/08/02 15:30:39]

You're right, my first stab of the description wasn't very accurate. The object
here is to record which context type name is used to create each canvas
rendering context. For example, for a 2D rendering context we'll record the name
"2d", while a 3D context can use a set of aliases like "webkit-3d", "webgl" and
"experimental-webgl". This happens at most once per <canvas> element on the
page.

Here's the blink-side patch that records the data if it helps:
https://codereview.chromium.org/21861002/

+    which names are being used to create canvas rendering contexts.
+  </summary>
+</histogram>
+
 <histogram name="Cellular.ActivationFailure">
   <summary>
     The count of cellular device activation failures (Chrome OS).
@@ -15547,6 +15554,13 @@ other types of suffix sets.
   <int value="1" label="Wiped out"/>
 </enum>
 
+<enum name="CanvasContextType" type="int">
+  <int value="0" label="2d"/>
+  <int value="1" label="webkit-3d"/>
+  <int value="2" label="experimental-webgl"/>
+  <int value="3" label="webgl"/>
+</enum>
+
 <enum name="ChannelLayout" type="int">
   <int value="0" label="CHANNEL_LAYOUT_NONE"/>
   <int value="1" label="CHANNEL_LAYOUT_UNSUPPORTED"/>
@@ -20826,6 +20840,12 @@ other types of suffix sets.
   <int value="2" label="Dismiss"/>
 </enum>
 
+<enum name="SimpleCache.FileDescriptorLimitStatus" type="int">
+  <int value="0" label="Unsupported"/>
+  <int value="1" label="Supported but failed"/>
+  <int value="2" label="Succeeded"/>
+</enum>
+
 <enum name="SimpleCacheHeaderSizeChange" type="int">
   <int value="0" label="Written for the first time"/>
   <int value="1" label="Rewritten with same size"/>
@@ -20834,12 +20854,6 @@ other types of suffix sets.
   <int value="4" label="Unexpected header stream write"/>
 </enum>
 
-<enum name="SimpleCache.FileDescriptorLimitStatus" type="int">
-  <int value="0" label="Unsupported"/>
-  <int value="1" label="Supported but failed"/>
-  <int value="2" label="Succeeded"/>
-</enum>
-
 <enum name="SimpleCacheIndexInitializeMethod" type="int">
   <int value="0" label="Directory Scan"/>
   <int value="1" label="Index File"/>