Make sure MHTML documents use the domain of the MHTML file.

https://bugs.webkit.org/show_bug.cgi?id=72445

Reviewed by Adam Barth.

Source/WebCore:

* dom/Document.h:
(WebCore::Document::setBaseURL):
* loader/FrameLoader.cpp:
(WebCore::FrameLoader::receivedFirstData):

LayoutTests:

* mhtml/check_domain-expected.txt: Added.
* mhtml/check_domain.mht: Added.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@100550 268f45cc-cd09-0410-ab3c-d52691b4dbfc