Clean up forward declarations in core/rendering and core/painting

These forward references are not needed anymore. After
this patch, core/rendering and core/painting has no unneeded
forward references any more.

This patch also move some forward declarations into correct
header files.

Review URL: https://codereview.chromium.org/713553002

git-svn-id: svn://svn.chromium.org/blink/trunk@185019 bbb929c8-8fbe-4397-9dbb-9b2b20218538