Add scroll-blocks-on support to devtools layers view

Source/core/rendering/compositing/CompositingReasonFinder.cpp

 // Copyright 2014 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 "config.h"
 #include "core/rendering/compositing/CompositingReasonFinder.h"
 
 #include "core/CSSPropertyNames.h"
 #include "core/dom/Document.h"
 #include "core/frame/FrameView.h"
@@ skipping 187 matching lines @@
         || renderer->document().documentElement()->renderer()->style()->scrollBlocksOn() == style->scrollBlocksOn());
 
     // When a scroll occurs, it's the union of all bits set on the target element's containing block
     // chain that determines the behavior.  Thus we really only need a new layer if this object contains
     // additional bits from those set by all objects in it's containing block chain.  But determining
     // this fully is probably more expensive than it's worth.  Instead we just have fast-paths here for
     // the most common cases of unnecessary layer creation.
     // Optimizing this fully would avoid layer explosion in pathological cases like '*' rules.
     // We could consider tracking the current state in CompositingRequirementsUpdater::update.
 
-    // Ensure iframes don't get composited when they require no more blocking than the root.
+    // Ensure iframes don't get composited when they require no more blocking than their parent.
     if (renderer->isRenderView()) {
         if (const FrameView* parentFrame = toRenderView(renderer)->frameView()->parentFrameView()) {
             if (const RenderView* parentRenderer = parentFrame->renderView()) {
                 // Does this frame contain only blocks-on bits already present in the parent frame?
                 if (!(style->scrollBlocksOn() & ~parentRenderer->style()->scrollBlocksOn()))
                     return false;
             }
+        } else {
+            // The root frame will either always already be composited, or compositing will be disabled.
+            // Either way, we don't need to require compositing for scroll blocks on.  This avoids
+            // enabling compositing by default, and avoids cluttering the root layers compositing reasons.
+            return false;
         }
     }
 
     return true;
 }
 
 }