Optimization for image decoding using Skia discardable memory

In the new Skia discardable memory path. We used to use heap memory to
decode. And then copy into the discardable memory allocated by Skia.
There's an opportunity for optimization. If we know that all data has been received
and the image is not a multi frame image. We can decode directly into
the memory given by Skia.

Review URL: https://codereview.chromium.org/141483004

git-svn-id: svn://svn.chromium.org/blink/trunk@166887 bbb929c8-8fbe-4397-9dbb-9b2b20218538