Apply attached patch and run the layout test.
Created attachment 42318 [details] Patch to demo the assert.
Which assert fires?
Created attachment 42319 [details] crash log (from assert while running layout test).
void FrameLoader::addData(const char* bytes, int length) { ASSERT(m_workingURL.isEmpty()); ASSERT(m_frame->document()); -> ASSERT(m_frame->document()->parsing()); This line is the assert that fires. It seems harmless in this case but I don't know completely.
Could this be a duplicate of any of the existing bugs with "multipart" in bug summary?
AP, good point. I found one that calls out the same assert, so I guess it is a dup of that. *** This bug has been marked as a duplicate of bug 14149 ***