Move paintSkBitmap and friends into NativeImageSkia.

These used to be static methods in ImageSkia, but they are more closely
connected to NativeImageSkia and fit better there.

BUG=242060

Review URL: https://chromiumcodereview.appspot.com/21022006

git-svn-id: svn://svn.chromium.org/blink/trunk@155265 bbb929c8-8fbe-4397-9dbb-9b2b20218538