Allow context-less document dispatching events.

This change allows context-less documents like ones from XHR to
dispatch events. More precicely, it allows event listeners in these documents
to react dispatched events.

For that purpose, it introduces Document::m_contextDocument
which points the document of the context where the document is created.

To prevent inline event handlers on context-less documents from being fired,
the change extracts the eligibility check from V8LazyEventLitener
to Document::allowInlineEventHandlers() and tightens it, so that it says no
if the owner document of the event-holding node doesn't have context (=Frame).

BUG=275285, 285357
TEST=event-on-created-document.html, event-on-xhr-document.html, import-events.html,
     sub-imports-onload.html
R=abarth@chromium.org, dglazkov@chromium.org, esprehn@chromium.org

Review URL: https://chromiumcodereview.appspot.com/23680010

git-svn-id: svn://svn.chromium.org/blink/trunk@157287 bbb929c8-8fbe-4397-9dbb-9b2b20218538