Early-out from the prepaint tree walk

third_party/WebKit/Source/core/paint/PrePaintTreeWalk.h

 // Copyright 2015 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.
 
 #ifndef PrePaintTreeWalk_h
 #define PrePaintTreeWalk_h
 
 #include "core/paint/PaintInvalidator.h"
 #include "core/paint/PaintPropertyTreeBuilder.h"
 
 namespace blink {
 
 class FrameView;
 class LayoutObject;
 struct PrePaintTreeWalkContext;
 
 // This class walks the whole layout tree, beginning from the root FrameView,
 // across frame boundaries. Helper classes are called for each tree node to
 // perform actual actions.  It expects to be invoked in InPrePaint phase.
 class PrePaintTreeWalk {
  public:
   void walk(FrameView& rootFrame);
 
  private:
-  void walk(FrameView&, const PrePaintTreeWalkContext&);
-  void walk(const LayoutObject&, const PrePaintTreeWalkContext&);
+  // Throttled rendering (see: FrameView::shouldThrottleRendering()) can prevent
+  // updating a subtree. We return true if the subtree was fully walked/updated,
+  // and false if the walk was interrupted by throttling.
+  bool walk(FrameView&, const PrePaintTreeWalkContext&);
+  bool walk(const LayoutObject&, const PrePaintTreeWalkContext&);
 
   PaintPropertyTreeBuilder m_propertyTreeBuilder;
   PaintInvalidator m_paintInvalidator;
 };
 
 }  // namespace blink
 
 #endif  // PrePaintTreeWalk_h