aura: Use platform independent skia::PlatformCanvas when possible.

skia/skia.gyp

Index: skia/skia.gyp
diff --git a/skia/skia.gyp b/skia/skia.gyp
index 85b34b877ed78ec49218ffa2e8c9d415729c0a32..3549a92c8d815f0f73d916b4ce02e3ee3b1a25b4 100644
--- a/skia/skia.gyp
+++ b/skia/skia.gyp
@@ -712,6 +712,7 @@
         'ext/platform_canvas.h',
         'ext/platform_canvas_linux.cc',
         'ext/platform_canvas_mac.cc',
+        'ext/platform_canvas_skia.cc',
         'ext/platform_canvas_win.cc',
         'ext/platform_device.cc',
         'ext/platform_device.h',
@@ -855,6 +856,15 @@
             ['include', 'ext/platform_canvas_linux\\.cc$'],
           ],
         }],
+        [ 'use_aura == 1 and use_canvas_skia == 1', {
+          'sources/': [
+            ['exclude', 'ext/platform_canvas_mac\\.cc$'],
+            ['exclude', 'ext/platform_canvas_linux\\.cc$'],
+            ['exclude', 'ext/platform_canvas_win\\.cc$'],
+          ],
+        }, { # use_aura == 0 and use_canvas_skia == 1
+          'sources/': [ ['exclude', 'ext/platform_canvas_skia\\.cc$'] ],
+        }],
         [ 'toolkit_uses_gtk == 1', {
           'dependencies': [
             '../build/linux/system.gyp:gdk',