Remove FrameLoader:m_isLoadingMainResource, since it is always

Remove FrameLoader:m_isLoadingMainResource, since it is always
false at the only point it is checked.
https://bugs.webkit.org/show_bug.cgi?id=83819

Reviewed by Adam Barth.

No new tests, no functionality change intended.

* loader/DocumentWriter.cpp:
(WebCore::DocumentWriter::end): Merge with endIfNotLoadingMainResource(), always end, assert that we're still attached.
* loader/DocumentWriter.h:
* loader/FrameLoader.cpp:
* loader/FrameLoader.h: Remove isLoadingMainResource(), didEndDocument(), and m_isLoadingMainResource.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@114062 268f45cc-cd09-0410-ab3c-d52691b4dbfc