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

ppapi/shared_impl/ppapi_preferences.cc

Index: ppapi/shared_impl/ppapi_preferences.cc
diff --git a/ppapi/shared_impl/ppapi_preferences.cc b/ppapi/shared_impl/ppapi_preferences.cc
index 3fd7731afaafcdee024c94a77727afcf8c46df3e..1db53c0aef4e6120bfc0dd3cd53a04df99f3b736 100644
--- a/ppapi/shared_impl/ppapi_preferences.cc
+++ b/ppapi/shared_impl/ppapi_preferences.cc
@@ -23,7 +23,8 @@ Preferences::Preferences(const webkit_glue::WebPreferences& prefs)
       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() {