Add UMA histogram to determine how often Stage3D could be used.

ppapi/shared_impl/ppapi_preferences.cc

 // Copyright (c) 2012 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.
 
 #include "ppapi/shared_impl/ppapi_preferences.h"
 
 namespace ppapi {
 
 Preferences::Preferences()
     : default_font_size(0),
       default_fixed_font_size(0),
       number_of_cpu_cores(0),
       is_3d_supported(true),
       is_stage3d_supported(false) {
 }
 
 Preferences::Preferences(const webkit_glue::WebPreferences& prefs)
     : standard_font_family_map(prefs.standard_font_family_map),
       fixed_font_family_map(prefs.fixed_font_family_map),
       serif_font_family_map(prefs.serif_font_family_map),
       sans_serif_font_family_map(prefs.sans_serif_font_family_map),
       default_font_size(prefs.default_font_size),
       default_fixed_font_size(prefs.default_fixed_font_size),
       number_of_cpu_cores(prefs.number_of_cpu_cores),
       is_3d_supported(prefs.flash_3d_enabled),
-      is_stage3d_supported(prefs.flash_stage3d_enabled) {
+      is_stage3d_supported(prefs.flash_stage3d_enabled),
+      is_webgl_supported(prefs.experimental_webgl_enabled) {
 }
 
 Preferences::~Preferences() {
 }
 
 }  // namespace ppapi