[RootLayerScrolling] Fix position of root scrolling layer for rtl.

In the non-root-layer case, the CLM's main graphics layer will have
a compositing container with a scroll origin, but in the root layer
case, it makes no sense to give the root graphics layer a non-zero
position.

This fixes most or all of the pixel diffs in the compositing/rtl/
layout tests with RLS enabled.  Those tests still need to be
rebaselined for text diffs.

BUG=542432
R=skobes@chromium.org

Change-Id: I1b37d8c96dd3328754770e4a7249c22b87550e14
Reviewed-on: https://chromium-review.googlesource.com/563487Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#485443}

third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.cpp

@@ -1211,7 +1211,7 @@ void CompositedLayerMapping::ComputeGraphicsLayerParentLocation(
         RoundedIntPoint(compositing_container->SubpixelAccumulation());
   } else if (compositing_container) {
     graphics_layer_parent_location = ancestor_compositing_bounds.Location();
-  } else {
+  } else if (!RuntimeEnabledFeatures::RootLayerScrollingEnabled()) {
     graphics_layer_parent_location =
         GetLayoutObject().View()->DocumentRect().Location();
   }