BitmapImage currentFrameSize() is not used, remove it

Clean up, and remove this unused function.

BUG=

Review URL: https://codereview.chromium.org/584113002

git-svn-id: svn://svn.chromium.org/blink/trunk@182350 bbb929c8-8fbe-4397-9dbb-9b2b20218538