Save a memory copy on Android for decoding images to discardable memory

On Android we can save copying the bitmap if the decoder writes directly
to the discardable memory.

All webkit_unit_tests passed with this change.

BUG=352570

Review URL: https://codereview.chromium.org/198603003

git-svn-id: svn://svn.chromium.org/blink/trunk@169457 bbb929c8-8fbe-4397-9dbb-9b2b20218538