Crash fix for multiple fullscreen layout tests.

Chromium r261087 resolved a "navigate away from fullscreen" bug, but
this started triggering crashes in some of the Blink layout tests due to
an ASSERT failure.  The ASSERTion assumes that a document will always be
active at the point when the exit fullscreen takes place.  However,
there are specific situations where the Document might be stopped before
fullscreen is exited.

This change simply changes the ASSERT to a return-early guard.  It also
re-enables the affected layout tests.

BUG=359067,356951,256870

Review URL: https://codereview.chromium.org/225263022

git-svn-id: svn://svn.chromium.org/blink/trunk@171069 bbb929c8-8fbe-4397-9dbb-9b2b20218538