Clear SkBitmap used to draw DragImage.

      SkBitmap::allocN32Pixels does not initialize the pixel memory, and we
may not draw to all of it (if, for example, an image decode fails,
nothing will be drawn).

BUG=450389

Review URL: https://codereview.chromium.org/901223002

git-svn-id: svn://svn.chromium.org/blink/trunk@189585 bbb929c8-8fbe-4397-9dbb-9b2b20218538