Add canvas context type histogram

tools/metrics/histograms/histograms.xml

 <!--
 Copyright 2013 The Chromium Authors. All rights reserved.
 Use of this source code is governed by a BSD-style license that can be
 found in the LICENSE file.
 -->
 
 <!--
 This file is used to generate a comprehensive list of Chrome histograms along
 with a detailed description for each histogram.
 
@@ skipping 1067 matching lines @@
 </histogram>
 
 <histogram name="Bluetooth.PairingResult" enum="BluetoothPairingResult">
   <summary>
     Records the result of pairing each Bluetooth Device. Used to understand
     whether we are having significant problems with Bluetooth pairing and seeing
     errors more commonly than we should.
   </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).
   </summary>
 </histogram>
 
 <histogram name="Cellular.ActivationTry">
   <summary>The count of cellular device activation tries (Chrome OS).</summary>
 </histogram>
 
@@ skipping 14442 matching lines @@
 <enum name="BooleanValid" type="int">
   <int value="0" label="Invalid"/>
   <int value="1" label="Valid"/>
 </enum>
 
 <enum name="BooleanWiped" type="int">
   <int value="0" label="Re-enabled"/>
   <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"/>
   <int value="2" label="CHANNEL_LAYOUT_MONO"/>
   <int value="3" label="CHANNEL_LAYOUT_STEREO"/>
   <int value="4" label="CHANNEL_LAYOUT_2_1"/>
   <int value="5" label="CHANNEL_LAYOUT_SURROUND"/>
   <int value="6" label="CHANNEL_LAYOUT_4POINT0"/>
   <int value="7" label="CHANNEL_LAYOUT_2_2"/>
   <int value="8" label="CHANNEL_LAYOUT_QUAD"/>
@@ skipping 5259 matching lines @@
   <int value="2" label="Extension re-enabled"/>
   <int value="3" label="Extension uninstalled"/>
 </enum>
 
 <enum name="SideloadWipeoutBubble" type="int">
   <int value="0" label="Learn more"/>
   <int value="1" label="Settings page"/>
   <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"/>
   <int value="2" label="Rewritten with larger size"/>
   <int value="3" label="Rewritten with smaller size"/>
   <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"/>
   <int value="2" label="New Cache"/>
 </enum>
 
 <enum name="SimpleCacheOpenEntryIndexState" type="int">
   <int value="0" label="No index"/>
   <int value="1" label="Hit"/>
   <int value="2" label="Miss"/>
@@ skipping 3196 matching lines @@
   <group name="Disabled" label="Neither extra webstore link is visible"/>
   <group name="FooterLink" label="Link in bottom right of footer"/>
   <group name="PlusIcon" label="Plus icon in apps page"/>
   <affected-histogram name="Extensions.AppLaunch"/>
   <affected-histogram name="NewTabPage.DefaultPageType"/>
 </fieldtrial>
 
 </fieldtrials>
 
 </histogram-configuration>