Initialize document m_URL & m_baseURL to blankURL.

This patch will ensure that m_URL and m_baseURL are initialized to
blankURL (about:blank) as is specified in [1]. This patch fixes the
tests in [2] and [3].

[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/2749803003
Cr-Commit-Position: refs/heads/master@{#457347}
24 files changed