RootLayerScrolling: fix PaintLayerClipperTest.

Configure the ClipRectsContext for the root layer to treat it as a
composited scrolling layer by not applying an overflow clip.

BUG=490942
R=chrishtr@chromium.org,skobes@chromium.org
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_layout_tests_slimming_paint_v2

Review-Url: https://codereview.chromium.org/2576883002
Cr-Commit-Position: refs/heads/master@{#438677}

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

@@ -27,6 +27,10 @@ TEST_F(PaintLayerClipperTest, LayoutSVGRoot) {
   PaintLayer* targetPaintLayer =
       toLayoutBoxModelObject(target->layoutObject())->layer();
   ClipRectsContext context(document().layoutView()->layer(), UncachedClipRects);
+  // When RLS is enabled, the LayoutView will have a composited scrolling layer,
+  // so don't apply an overflow clip.
+  if (RuntimeEnabledFeatures::rootLayerScrollingEnabled())
+    context.setIgnoreOverflowClip();
   LayoutRect layerBounds;
   ClipRect backgroundRect, foregroundRect;
   targetPaintLayer->clipper().calculateRects(