[CSS Shapes] Import and fix polygon shapes reftests

Import these polygon shapes reftests and fix them.
Note that these tests should be imported at some point.

This is based on http://trac.webkit.org/changeset/172973 by Zoltan Horvath <zoltan@webkit.org>.

From the WebKit patch commit log:
We didn't respect the shape-margin before a positioned polygon, since we checked
the shape-margin for only 1 vertex for a polygon edge. This patch fixes the behavior,
removes the tests from the skipped list and updates 1 test, which was incorrect.

Review URL: https://codereview.chromium.org/510513002

git-svn-id: svn://svn.chromium.org/blink/trunk@181504 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Clean up forward declarations in Source/core/css

These forward references are not needed anymore.

Review URL: https://codereview.chromium.org/541383003

git-svn-id: svn://svn.chromium.org/blink/trunk@181505 bbb929c8-8fbe-4397-9dbb-9b2b20218538