Add a StaleTestExpectations file.

Move all the platform-specific entries with webkit.org bugs to it.
See the bug for rationale.

BUG=308779
R=adamk@chromium.org

Review URL: https://codereview.chromium.org/27812002

git-svn-id: svn://svn.chromium.org/blink/trunk@160121 bbb929c8-8fbe-4397-9dbb-9b2b20218538