[RLS] Document's main GraphicsLayer size should not include overflow.

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/paint/PaintLayer.h"
 
 #include "core/html/HTMLIFrameElement.h"
 #include "core/layout/LayoutBoxModelObject.h"
 #include "core/layout/LayoutTestHelper.h"
 #include "core/layout/LayoutView.h"
@@ skipping 56 matching lines @@
       "  <div style='width: 1000px; height: 500px; background: lightgray'>"
       "  </div>"
       "</div>");
 
   PaintLayer* parentLayer =
       toLayoutBoxModelObject(getLayoutObjectByElementId("parent"))->layer();
   EXPECT_EQ(LayoutRect(0, 0, 784, 500),
             parentLayer->boundingBoxForCompositing());
 }
 
+TEST_P(PaintLayerTest, RootLayerCompositedBounds) {
+  setBodyInnerHTML(
+      "<style> body { width: 1000px; height: 1000px; margin: 0 } </style>");
+  EXPECT_EQ(RuntimeEnabledFeatures::rootLayerScrollingEnabled()
+                ? LayoutRect(0, 0, 800, 600)
+                : LayoutRect(0, 0, 1000, 1000),
+            layoutView().layer()->boundingBoxForCompositing());
+}
+
 TEST_P(PaintLayerTest, PaintingExtentReflection) {
   setBodyInnerHTML(
       "<div id='target' style='background-color: blue; position: absolute;"
       "    width: 110px; height: 120px; top: 40px; left: 60px;"
       "    -webkit-box-reflect: below 3px'>"
       "</div>");
 
   PaintLayer* layer =
       toLayoutBoxModelObject(getLayoutObjectByElementId("target"))->layer();
   EXPECT_EQ(
@@ skipping 628 matching lines @@
   EXPECT_EQ(LayoutPoint(-150, 50), spanner->location());
   EXPECT_EQ(LayoutPoint(100, 100), extraLayer->location());
   // -60 = 2nd-column-x(40) - scroll-offset-x(200) + x-location(100)
   // 20 = y-location(100) - column-height(80)
   EXPECT_EQ(LayoutPoint(-60, 20),
             extraLayer->visualOffsetFromAncestor(columns));
   EXPECT_EQ(LayoutPoint(-150, 50), spanner->visualOffsetFromAncestor(columns));
 }
 
 }  // namespace blink