Invalidate outstanding checkpoints for the background input stream and preload scanner
https://bugs.webkit.org/show_bug.cgi?id=110537

Reviewed by Adam Barth.

fast/tokenizer/write-before-load.html caused duplicate didFailSpeculation() calls for the same checkpoint.
One was triggered by processParsedChunkFromBackgroundParser and the other by resumeParsingAfterScriptExecution.
This change ensures that after we send a checkpoint, it is cleared so it won't be sent again until the next
processParsedChunkFromBackgroundParser().

No new tests because no new functionality.

* html/parser/BackgroundHTMLInputStream.cpp:
(WebCore::BackgroundHTMLInputStream::rewindTo):
* html/parser/HTMLDocumentParser.cpp:
(WebCore::HTMLDocumentParser::didFailSpeculation):
* html/parser/HTMLPreloadScanner.cpp:
(WebCore::TokenPreloadScanner::rewindTo):


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@143685 268f45cc-cd09-0410-ab3c-d52691b4dbfc