Prevent SkPictureUtils::GatherPixelRefs from generating bitmaps from gradient shaders.
Prevent SkPictureUtils::GatherPixelRefs from generating bitmaps from gradient
shaders.

BUG=https://code.google.com/p/chromium/issues/detail?id=173657
Review URL: https://codereview.appspot.com/7220074

csulmone/skia,csulmone/skia,csulmone/skia,csulmone/skia