roll skia to 2691

need to define SK_ENABLE_FAST_SHADERMASK to opt-in to faster shadermask
Review URL: http://codereview.chromium.org/8572025

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@110296 0039d316-1c4b-4281-b951-d872f2087c98

DEPS

@@ -28,7 +28,7 @@ vars = {
   "libvpx_revision": "109236",
   "ffmpeg_revision": "109946",
   "sfntly_revision": "98",
-  "skia_revision": "2681",
+  "skia_revision": "2692",
   "v8_revision": "9989",
   "webrtc_revision": "946",
   "jsoncpp_revision": "246",