Bug 1650719 - Don't lose the rect offset from the composition bounds. r=kats

This offset was reset in one of the two codepaths of D80723, but turns
out it's important for same-origin documents that are also
root-content-documents, like about: pages in the parent process, which
happened to hit the codepath that did _not_ reset it. :(

Differential Revision: https://phabricator.services.mozilla.com/D84584

layout/base/nsLayoutUtils.cpp

@@ -8732,8 +8732,8 @@ bool nsLayoutUtils::UpdateCompositionBoundsForRCDRSF(
                             shouldSubtractDynamicToolbar)) {
     return false;
   }
-  aCompBounds = ParentLayerRect(ViewAs<ParentLayerPixel>(
-      LayoutDeviceIntRect(LayoutDeviceIntPoint(), contentSize),
+  aCompBounds.SizeTo(ViewAs<ParentLayerPixel>(
+      LayoutDeviceSize(contentSize),
       PixelCastJustification::LayoutDeviceIsParentLayerForRCDRSF));
   return true;
 }