Have DOMImplementation::document() return reference.
    
Have DOMImplementaion::document() return reference
as m_document is a stored as a reference.

As this function is used in V8, new IDL attribute
[SetWrapperReferenceFromReference] has been added, which supports document() returning reference.

New binding test has been added for this attribute.

Review URL: https://codereview.chromium.org/140653013

git-svn-id: svn://svn.chromium.org/blink/trunk@167755 bbb929c8-8fbe-4397-9dbb-9b2b20218538