Add a gyp variable to turn on accelerated 2d canvas build time flags

build/features_override.gypi

Index: build/features_override.gypi
diff --git a/build/features_override.gypi b/build/features_override.gypi
index 1b7774761e11637564db3b9dc52c7f98b7c55473..c6d9dc97c71dc640a143303f29d99b7745f57f2d 100644
--- a/build/features_override.gypi
+++ b/build/features_override.gypi
@@ -55,10 +55,12 @@
     # through GYP_DEFINES.
     'variables': {
       'use_accelerated_compositing%': 0,
+      'accelerated_2d_canvas%': 0,
       'enable_svg%': 1,
       'enable_touch_events%': 1,
     },
     'use_accelerated_compositing%': '<(use_accelerated_compositing)',
+    'accelerated_2d_canvas%': '<(accelerated_2d_canvas)',
     'enable_svg%': '<(enable_svg)',
     'enable_touch_events%': '<(enable_touch_events)',
     'conditions': [
@@ -69,6 +71,11 @@
         ],
         'use_accelerated_compositing': 1,
       }],
+      ['accelerated_2d_canvas==1', {
+        'feature_defines': [
+          'ENABLE_ACCELERATED_2D_CANVAS=1',
+        ]
+      }]
     ],
     # TODO: If the need arises, create a mechanism that will intelligently
     # merge the lists rather than replace one with the other. This may