[CSS Shapes] Image valued shape size and position should conform to the spec

Implement image valued shape-outside scaling and translation per the spec, http://dev.w3.org/csswg/css-shapes/#shapes-from-image:

"The image is sized and positioned as if it were a replaced element whose specified width and height are the same as the elements used content box size."

This change doesn't completely fulfill the spec, it's limited to image elements.

BUG=311388

Review URL: https://codereview.chromium.org/145603005

git-svn-id: svn://svn.chromium.org/blink/trunk@165671 bbb929c8-8fbe-4397-9dbb-9b2b20218538