Prefer pre-parsed Document::m_url over Document::m_documentURI string

m_documentURI used to be writable from JavaScript (before
http://trac.webkit.org/changeset/146580) and Objective-C (before Blink).

Today, it's always set to m_url.string(), so we can instead use
[ImplementedAs] in Document.idl, rely on KURL's implicit String conversion
for the IDL attribute, and use m_url as the fallback base URL in
Document::updateBaseURL().

R=mkwst@chromium.org

Review URL: https://codereview.chromium.org/122683008

git-svn-id: svn://svn.chromium.org/blink/trunk@164477 bbb929c8-8fbe-4397-9dbb-9b2b20218538