Clean up forward declarations in Source/platform

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

This patch is the second part of
https://codereview.chromium.org/531983002/.

Review URL: https://codereview.chromium.org/531983002

git-svn-id: svn://svn.chromium.org/blink/trunk@181271 bbb929c8-8fbe-4397-9dbb-9b2b20218538