Set default document urls to 'about:blank'.

The patch ensures that the document urls are set to 'about:blank', as is
specified in [1], if a url has not been specified. It fixes the tests in
[2] and [3].

The functionality is implemented by binding the document urls to
'about:blank' if m_url is empty. To achieve this behavior the
Document::urlForBinding() function was added. The choice for a binding
was made rather than not using an intial value of 'about:blank' for the
document urls, as other parts of the code rely on the document urls
initially being empty.

More details regarding the binding can be found under:
https://codereview.chromium.org/2749803003/#msg37

Additionally the platform specific dom-interfaces test have been
removed, as they were identical, making platform specific expectations
superfluous. There is now a single interfaces test located at
third_party/WebKit/LayoutTests/external/wpt/dom/interfaces.html

[1] https://dom.spec.whatwg.org/#interface-document
[2] http://w3c-test.org/dom/nodes/DOMImplementation-createDocument.html
[3] http://w3c-test.org/dom/nodes/DOMImplementation-createHTMLDocument.html

BUG=563986

Review-Url: https://codereview.chromium.org/2768373005
Cr-Commit-Position: refs/heads/master@{#459624}
24 files changed