[RLS] Skip compositing input step if a scroll can be applied directly.

This re-introduces the fast path implemented in PLC::FrameViewDidScroll.

Test coverage: all layout tests which use main-thread scrolling of a
composited layer (e.g. using window.scrollBy or gestures on scrollbars).

Bug: 770343
Cq-Include-Trybots: master.tryserver.blink:linux_trusty_blink_rel;master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2
Change-Id: I93a43072d19871611fa8845758410b094d67a8f7
Reviewed-on: https://chromium-review.googlesource.com/923089Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Chris Harrelson <chrishtr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537143}

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

@@ -2063,8 +2063,16 @@ void PaintLayerScrollableArea::UpdateCompositingLayersAfterScroll() {
     DCHECK(Layer()->HasCompositedLayerMapping());
     Layer()->GetCompositedLayerMapping()->SetNeedsGraphicsLayerUpdate(
         kGraphicsLayerUpdateSubtree);
-    compositor->SetNeedsCompositingUpdate(
-        kCompositingUpdateAfterGeometryChange);
+
+    ScrollingCoordinator* scrolling_coordinator = GetScrollingCoordinator();
+    bool handled_scroll =
+        Layer()->IsRootLayer() && scrolling_coordinator &&
+        scrolling_coordinator->ScrollableAreaScrollLayerDidChange(this);
+
+    if (!handled_scroll) {
+      compositor->SetNeedsCompositingUpdate(
+          kCompositingUpdateAfterGeometryChange);
+    }
 
     // If we have fixed elements and we scroll the root layer in RLS we might
     // change compositing since the fixed elements might now overlap a