Bug 1969100 - Don't ignore UA-widget nodes for paragraph selection. r=sefeng a=RyanVM

Not sure how useful adding a test for this really is. The XML pretty
printer is pretty much the only UA widget that has some paragraph
content, and setting it up is non-trivial. If you insist I can look into
adding a test tho.

layout/generic/nsIFrame.cpp

@@ -5512,7 +5512,8 @@ static bool IsRelevantBlockFrame(const nsIFrame* aFrame) {
     return false;
   }
   if (aFrame->GetContent() &&
-      aFrame->GetContent()->IsInNativeAnonymousSubtree()) {
+      aFrame->GetContent()->IsInNativeAnonymousSubtree() &&
+      !aFrame->GetContent()->HasBeenInUAWidget()) {
     // This helps skipping things like scrollbar parts.
     return false;
   }