Backout changeset ef47dbb6313a (bug 539356) because of performance and correctness regressions

layout/base/nsPresContext.cpp

@@ -1179,18 +1179,6 @@ nsPresContext::GetParentPresContext()
         return f->PresContext();
     }
   }
-  // Not sure if this is always strictly the parent, but it works for GetRootPresContext
-  // where the current pres context has no frames.
-  nsIDocument *doc = Document();
-  if (doc) {
-    doc = doc->GetParentDocument();
-    if (doc) {
-      shell = doc->GetShell();
-      if (shell) {
-        return shell->GetPresContext();
-      }
-    }
-  }
   return nsnull;
 }
 
@@ -1215,7 +1203,7 @@ nsPresContext::GetRootPresContext()
   nsPresContext* pc = this;
   for (;;) {
     nsPresContext* parent = pc->GetParentPresContext();
-    if (!parent || parent == pc)
+    if (!parent)
       break;
     pc = parent;
   }