Add null check in ChromeClientImpl::postAccessibilityNotification().

Because AXObjectCache::accessibilityEnabled() always returns true in the shared worker process,
ChromeClientImpl::postAccessibilityNotification() is never called in the process.
So we don't need to check m_webView->client() now.

But we are planing to move SharedWorker to the renderer process.
http://crbug.com/327256
ChromeClientImpl::postAccessibilityNotification() will be called in the shared worker's WebFrame
when the accessibility feature is enabled.
But m_webView->client() is NULL in the shared worker's WebFrame.

So we have to check it.

BUG=327256

Review URL: https://codereview.chromium.org/134643019

git-svn-id: svn://svn.chromium.org/blink/trunk@165606 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Add support for DOM4's XMLDocument interface

Add support for DOM4's XMLDocument interface:
http://dom.spec.whatwg.org/#xmldocument

DOMImplementation.createDocument() now returns an XMLDocument instead of a
Document as per:
http://dom.spec.whatwg.org/#dom-domimplementation-createdocument

DOMParser.parseFromString() now returns an XMLDocument instead of a Document
when the input type is an xml type, as per:
http://domparsing.spec.whatwg.org/#dom-domparser-parsefromstring

This behavior is consistent with Firefox 26 and IE 11.
The compatibility risk is low since an XMLDocument is identical to a Document.

R=tkent, arv
BUG=238366
BUG=238372
TEST=fast/dom/DOMImplementation/createDocument-XMLDocument.html

Review URL: https://codereview.chromium.org/144063004

git-svn-id: svn://svn.chromium.org/blink/trunk@165607 bbb929c8-8fbe-4397-9dbb-9b2b20218538