Clean up forward declarations in Source/core/rendering

These forward references are not needed anymore. After
this patch, Source/core/rendering has no unneeded forward
references anymore.

Review URL: https://codereview.chromium.org/569743004

git-svn-id: svn://svn.chromium.org/blink/trunk@182270 bbb929c8-8fbe-4397-9dbb-9b2b20218538