Do not assume that v8::Context::GetEntered returns a valid context

messageHandlerInMainThread may be called during context initialization,
at which time there is no Entered context. Bail out early if that
happens, as it won't be possible to determine the "first" window.

Also fix up other callers of firstDOMWindow() and activeDOMWindow() to
assume they're non-null.

R=abarth@chromium.org, haraken@chromium.org, abarth
BUG=302041

Review URL: https://codereview.chromium.org/25347003

git-svn-id: svn://svn.chromium.org/blink/trunk@158663 bbb929c8-8fbe-4397-9dbb-9b2b20218538