Deferred image decoder generates new SkBitmap in progressive decode

A new SkBitmap needs to be generated for each progressive decode.
Otherwise Skia will use the older bitmap from the cache.

BUG=318490

Review URL: https://codereview.chromium.org/113893004

git-svn-id: svn://svn.chromium.org/blink/trunk@164063 bbb929c8-8fbe-4397-9dbb-9b2b20218538