Make MessagePort a normal ActiveDOMObject

This removes special treatment in ExecutionContext, V8GCController, and
code_generator_v8.pm, as well as removing some boilerplate from MessagePort
itself.

The one functional difference is that MessageEvent dispatching from ports
now happens in a new task for each port being delivered to, rather than in
a single combined task for all ports running in an ExecutionContext. I don't
expect this to come up much in practice, though.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/60233009

git-svn-id: svn://svn.chromium.org/blink/trunk@162146 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove TreatNullAs=NullString for <frame>, <frameset> and <iframe>

http://www.whatwg.org/specs/web-apps/current-work/multipage/obsolete.html#htmlframeelement
http://www.whatwg.org/specs/web-apps/current-work/multipage/obsolete.html#htmlframesetelement
http://www.whatwg.org/specs/web-apps/current-work/multipage/the-iframe-element.html#htmliframeelement
http://www.whatwg.org/specs/web-apps/current-work/multipage/obsolete.html#HTMLIFrameElement-partial

Per spec the longDesc attribute on HTMLFrameElement and
HTMLIFrameElement should be reflected as a URL, so make it so.

HTMLIFrameElement.sandbox is readonly DOMSettableTokenList in the spec,
so leave it with [TreatNullAs=NullString] until that is fixed.

The tests for the updated properties pass in Firefox Nightly, IE11
Release Preview and Opera 12.16, with these exceptions:

IE11 throws exceptions for HTMLFrameElement.scrolling,
HTMLIFrameElement.align and HTMLIFrameElement.scrolling, reflects
HTMLIFrameElement.width/height as the empty string and does not support
HTMLIFrameElement.srcdoc.

Opera does not support HTMLIFrameElement.srcdoc.

Note that no other browser supports the location attribute, but that has
not been touched here.

BUG=310298

Review URL: https://codereview.chromium.org/68503013

git-svn-id: svn://svn.chromium.org/blink/trunk@162147 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unused V8ThrowException::createDOMException() helper.

Followup minor issue from crrev.com/68563003 and remove an unused helper
that also supplies an empty creation context.

R=ch.dumez
BUG=

Review URL: https://codereview.chromium.org/70103024

git-svn-id: svn://svn.chromium.org/blink/trunk@162148 bbb929c8-8fbe-4397-9dbb-9b2b20218538