Remove pre-scaling code from NativeImageSkia.

This is properly handled by Skia.

After this CL, NativeImageSkia is a very thin wrapper around an SkBitmap.

BUG=391051

Review URL: https://codereview.chromium.org/685333003

git-svn-id: svn://svn.chromium.org/blink/trunk@184668 bbb929c8-8fbe-4397-9dbb-9b2b20218538