Remove WorkerScriptController::controllerForContext()

It's no longer needed. We can replace call sites of WorkerScriptController::controllerForContext()
with other existing methods.

BUG=341032
R=jochen@chromium.org

Review URL: https://codereview.chromium.org/196543002

git-svn-id: svn://svn.chromium.org/blink/trunk@169098 bbb929c8-8fbe-4397-9dbb-9b2b20218538