Ignore V8's message handler that is invoked while we're initializing a window

messageHandlerInMainThread can be invoked while we're initializing a window (See stack traces in the bugs). In that half-baked situation, we don't have a valid context nor a valid world, so we should return immediately.

BUG=345014,352179
NOTRY=true

Review URL: https://codereview.chromium.org/203123006

git-svn-id: svn://svn.chromium.org/blink/trunk@169518 bbb929c8-8fbe-4397-9dbb-9b2b20218538