Add type cast functions for StyleImage subclasses, and use them.

We should avoid bare static_cast to detect bad cast earlier.

BUG=309516

Review URL: https://codereview.chromium.org/69673002

git-svn-id: svn://svn.chromium.org/blink/trunk@161748 bbb929c8-8fbe-4397-9dbb-9b2b20218538