Extract Document::allowInlineEventHandlers()

This change extracts Document::allowInlineEventHandlers()
from V8LazyEventListeners. This is a preparation for
crbug.com/275285

BUG=275285
R=dglazkov@chromium.org,tkent@chromium.org,haraken@chromium.org
TEST=none

Review URL: https://chromiumcodereview.appspot.com/23583009

git-svn-id: svn://svn.chromium.org/blink/trunk@156896 bbb929c8-8fbe-4397-9dbb-9b2b20218538