Fix crash in isDocumentDoneLoading()
Fix crash in isDocumentDoneLoading()

Document::loader() might be null if re-entrancy is involved.

BUG=357570

Review URL: https://codereview.chromium.org/222433003

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@170693 bbb929c8-8fbe-4397-9dbb-9b2b20218538

primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs,primiano/blink-gitcs