Clean up forward declarations in Source/core

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

Review URL: https://codereview.chromium.org/732803003

git-svn-id: svn://svn.chromium.org/blink/trunk@185404 bbb929c8-8fbe-4397-9dbb-9b2b20218538