Simplify createShapeForImage handling of image pointers

StyleImage::image can be used to get a cached image just like it can be
used to get a generated image. So we can just have one RefPtr and remove
the raw pointer, simplifiying the code immensely.

Review URL: https://codereview.chromium.org/316983003

git-svn-id: svn://svn.chromium.org/blink/trunk@175516 bbb929c8-8fbe-4397-9dbb-9b2b20218538