Bug 1404140 - Return early when trying to flush to get the style of a disconnected element. r=heycam

We just don't return a styles for them anyway, and this simplifies the following
patches a bit.

Differential Revision: https://phabricator.services.mozilla.com/D40297

layout/style/nsComputedDOMStyle.cpp

@@ -828,6 +828,13 @@ void nsComputedDOMStyle::UpdateCurrentStyleSources(bool aNeedsLayoutFlush) {
     return;
   }
 
+  // We don't return styles for disconnected elements anymore, so don't go
+  // through the trouble of flushing or what not.
+  if (!mElement->IsInComposedDoc()) {
+    ClearComputedStyle();
+    return;
+  }
+
   // TODO(emilio): We may want to handle a few special-cases here:
   //
   //  * https://github.com/w3c/csswg-drafts/issues/1964