Remove currentWorldContextOrMainWorldContext()

currentWorldContextOrMainWorldContext() is doing two wrong things:

(1) It returns a main world context when the frame is already detached.
(2) It retrieves a context associated with the current world.

This CL replaces currentWorldContextOrMainWorldContext() with toV8Context(Frame*) and fixes the above two issues (with a couple of FIXMEs).

BUG=341032

Review URL: https://codereview.chromium.org/156283003

git-svn-id: svn://svn.chromium.org/blink/trunk@167011 bbb929c8-8fbe-4397-9dbb-9b2b20218538