messageHandlerInMainThread shouldn't create a wrapper during context initialization

This is a regression caused by r155454.

We need to skip creating a wrapper of an error event object
if messageHandlerInMainThread is called during context initialization.
It's not allowed to create a wrapper during context initialization.

I couldn't reproduce the crash in layout tests because
the crash disappears when I use testRunner.waitUntilDone()...

TEST=ManualTests/error-event-during-context-initialization-crash.html
BUG=269709
R=mkwst@chromium.org

Review URL: https://codereview.chromium.org/22273008

git-svn-id: svn://svn.chromium.org/blink/trunk@155745 bbb929c8-8fbe-4397-9dbb-9b2b20218538