Removed RenderLayer unused members

The function enclosingScrollableLayer along with several #includes
and forward declarations where unused after moving the code around.

BUG=260898

Review URL: https://codereview.chromium.org/128003002

git-svn-id: svn://svn.chromium.org/blink/trunk@164678 bbb929c8-8fbe-4397-9dbb-9b2b20218538