Oilpan: Use weak processing to manage clean-up of ScreenOrientationControllers.

R=haraken@chromium.org
BUG=

Review URL: https://codereview.chromium.org/265843002

git-svn-id: svn://svn.chromium.org/blink/trunk@173084 bbb929c8-8fbe-4397-9dbb-9b2b20218538