Fixed position elements should layout relative to transformed container

The original layer coordinates conversion code contains a bug that it forgets
to return for the simple case where the fixed position container found is
already the ancestor layer wanted, and the code falls through to the special
path intended for RenderFlowThread.

R=jchaffraix@chromium.org
BUG=243967

Review URL: https://codereview.chromium.org/16057002

git-svn-id: svn://svn.chromium.org/blink/trunk@153254 bbb929c8-8fbe-4397-9dbb-9b2b20218538