[RLS] Remove !RLS code in paint_layer_clipper & root_scroller_controller

This patch removes the !RLS codepaths in paint layer clipper and root
scroller controller. Now that Root Layer Scrolling (RLS) is enabled
by default, this code is no longer needed.

Bug: 823365
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I55e925d572dfbcee2cb4db53c73ca9f841a4d01b
Reviewed-on: https://chromium-review.googlesource.com/1072632Reviewed-by: Steve Kobes <skobes@chromium.org>
Reviewed-by: Chris Harrelson <chrishtr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#561674}

third_party/blink/renderer/core/page/scrolling/root_scroller_controller.cc

@@ -85,10 +85,6 @@ PaintLayerScrollableArea* GetScrollableArea(const Element& element) {
     if (!frame_view)
       return nullptr;
 
-    // Not supported with RLS turned off.
-    if (!RuntimeEnabledFeatures::RootLayerScrollingEnabled())
-      return nullptr;
-
     return ToPaintLayerScrollableArea(
         frame_view->LayoutViewportScrollableArea());
   }

third_party/blink/renderer/core/paint/paint_layer_clipper.cc

@@ -418,13 +418,6 @@ void PaintLayerClipper::CalculateRects(
 void PaintLayerClipper::CalculateClipRects(const ClipRectsContext& context,
                                            ClipRects& clip_rects) const {
   const LayoutBoxModelObject& layout_object = layer_.GetLayoutObject();
-  if (!layer_.Parent() &&
-      !RuntimeEnabledFeatures::RootLayerScrollingEnabled()) {
-    // The root layer's clip rect is always infinite.
-    clip_rects.Reset(LayoutRect(LayoutRect::InfiniteIntRect()));
-    return;
-  }
-
   bool is_clipping_root = &layer_ == context.root_layer;
 
   if (is_clipping_root && !context.ShouldRespectRootLayerClip()) {