https://bugs.webkit.org/show_bug.cgi?id=107664

Reviewed by Adam Barth.

The HTMLDocumentParser can be "supposed" to use a background parser
but not end up ever doing so for blank documents.  It's important
that we spin the forground parser at least once, so we end up
actually creating the about:blank document.

shouldUseThreading() means that threading is enabled and we should use it if we can.
m_haveBackgroundParser means that we actually are already using threading.
When we add full document.write support this will get a bit trickier still as
we'll need to be using both the foreground and background parsers.

This fixes about 20 tests in fast/frames and many others outside of fast/frames. :)

* html/parser/HTMLDocumentParser.cpp:
(WebCore::HTMLDocumentParser::prepareToStopParsing):
(WebCore::HTMLDocumentParser::resumeParsingAfterYield):
(WebCore::HTMLDocumentParser::finish):
(WebCore::HTMLDocumentParser::lineNumber):
(WebCore::HTMLDocumentParser::textPosition):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@140559 268f45cc-cd09-0410-ab3c-d52691b4dbfc