Remove ScriptController::contextForWorld() and ScriptController::mainWorldContext()

We don't want to have multiple ways to extract contexts.
We want to unify similar methods to toV8Context().

BUG=341032

Review URL: https://codereview.chromium.org/165083003

git-svn-id: svn://svn.chromium.org/blink/trunk@167266 bbb929c8-8fbe-4397-9dbb-9b2b20218538