Non-composited, fixed position should trigger main thread scrolling

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 131 matching lines @@
 
         if (const RenderLayer* scrollingAncestor = layer->ancestorScrollingLayer()) {
             if (scrollingAncestor->needsCompositedScrolling() && layer->scrollParent())
                 directReasons |= CompositingReasonOverflowScrollingParent;
         }
 
         if (layer->needsCompositedScrolling())
             directReasons |= CompositingReasonOverflowScrollingTouch;
     }
 
-    if (requiresCompositingForPositionFixed(renderer))
+    if (requiresCompositingForPositionFixed(layer))
         directReasons |= CompositingReasonPositionFixed;
 
     directReasons |= renderer->additionalCompositingReasons();
 
     ASSERT(!(directReasons & CompositingReasonComboAllStyleDeterminedReasons));
     return directReasons;
 }
 
 bool CompositingReasonFinder::requiresCompositingForAnimation(RenderStyle* style) const
 {
     if (style->subtreeWillChangeContents())
         return style->isRunningAnimationOnCompositor();
 
     return style->shouldCompositeForCurrentAnimations();
 }
 
-bool CompositingReasonFinder::requiresCompositingForPositionFixed(RenderObject* renderer) const
+bool CompositingReasonFinder::requiresCompositingForPositionFixed(const RenderLayer* layer) const
 {
     if (!(m_compositingTriggers & ViewportConstrainedPositionedTrigger))
         return false;
     // Don't promote fixed position elements that are descendants of a non-view container, e.g. transformed elements.
     // They will stay fixed wrt the container rather than the enclosing frame.
-    return renderer->style()->position() == FixedPosition
-        && renderer->container() == &m_renderView
-        && m_renderView.frameView()->isScrollable();
+    return layer->scrollsWithViewport() && m_renderView.frameView()->isScrollable();
 }
 
 }