Reviewed by David Levin.

        Add tests for FileWriterSync
        https://bugs.webkit.org/show_bug.cgi?id=50720

        * fast/filesystem/resources/file-writer-sync-truncate-extend.js: Added.
        * fast/filesystem/workers/file-writer-sync-truncate-extend.html: Added.
        * fast/filesystem/workers/file-writer-sync-truncate-extend-expected.txt: Added.
        * fast/filesystem/resources/file-writer-sync-write-overlapped.js: Added.
        * fast/filesystem/workers/file-writer-sync-write-overlapped.html: Added.
        * fast/filesystem/workers/file-writer-sync-write-overlapped-expected.txt: Added.
        * fast/filesystem/resources/file-writer-utils.js:
        (assert): Make assert throw, rather than continue.
        (cleanUp): Always call finishJSTest, even without fileEntryForCleanup.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@75864 268f45cc-cd09-0410-ab3c-d52691b4dbfc