[Android] ImageBuffer toDataURL("image/webp") has RGBA pixel input

On Android, a premultiplied ImageBuffer source has RGBA pixels. Import
its bitmap for encoding in that form, rather than BGRA.

Covered by fast/canvas/canvas-toDataURL-webp.html

BUG=421454

Review URL: https://codereview.chromium.org/645333002

git-svn-id: svn://svn.chromium.org/blink/trunk@183712 bbb929c8-8fbe-4397-9dbb-9b2b20218538