Bug 1405547: Account for mid-unbind conditions in the BitsArePropagated assertion too. r=heycam

I'm not adding a crashtest because the one provided is kinda unreliable, and I
suck and I wasn't able to find a better one.

MozReview-Commit-ID: CmaF0NO4Dff

--HG--
extra : rebase_source : 5d324499b1a258f9d73266c06adbc46ae98db896

dom/base/Element.cpp

@@ -4575,6 +4575,7 @@ NoteDirtyElement(Element* aElement, uint32_t aBits)
              nsContentUtils::ContentIsFlattenedTreeDescendantOfForStyle(
                aElement, doc->GetServoRestyleRoot()));
   MOZ_ASSERT(aElement == doc->GetServoRestyleRoot() ||
+             !doc->GetServoRestyleRoot()->IsElement() ||
              BitsArePropagated(parent, aBits, doc->GetServoRestyleRoot()));
   MOZ_ASSERT(doc->GetServoRestyleRootDirtyBits() & aBits);
 }