Issue #618 - (async) Keep track of script modes in a single mode state.

This simplifies handling of combinations of async/defer by assigning one and
only one state to scripts.
If async then always async > if defer or module then defer > otherwise blocking.

dom/script/ModuleLoadRequest.cpp

@@ -63,6 +63,7 @@ ModuleLoadRequest::ModuleLoadRequest(nsIURI* aURI,
   MOZ_ASSERT(mVisitedSet->Contains(aURI));
 
   mIsInline = false;
+  mScriptMode = aParent->mScriptMode;
 }
 
 void ModuleLoadRequest::Cancel()