Fix the bug that negative outline-offset is covered up by composited

third_party/WebKit/Source/platform/graphics/CompositingReasons.h

 // 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.
 
 #ifndef CompositingReasons_h
 #define CompositingReasons_h
 
 #include "platform/PlatformExport.h"
 #include "wtf/Allocator.h"
 #include <stdint.h>
@@ skipping 75 matching lines @@
 const uint64_t CompositingReasonLayerForClippingMask = UINT64_C(1) << 46;
 const uint64_t CompositingReasonLayerForScrollingBlockSelection = UINT64_C(1)
                                                                   << 47;
 
 // Composited elements with inline transforms trigger assumed overlap so that
 // we can update their transforms quickly.
 const uint64_t CompositingReasonInlineTransform = UINT64_C(1) << 48;
 
 const uint64_t CompositingReasonCompositorProxy = UINT64_C(1) << 49;
 
+// Composited layer painted on top of all other layers as decoration
+const uint64_t CompositingReasonLayerForDecoration = UINT64_C(1) << 50;

[flackr, 2016/10/24 17:19:30]

We don't need to add a compositing reason for this - we should never composite
something just because it has a negative-offset outline, it will just create an
extra layer when composited if we need to for  the outline.

[yigu, 2016/10/24 20:31:36]

Will add a compositing reason as discussed.

+
 // Various combinations of compositing reasons are defined here also, for more
 // intutive and faster bitwise logic.
 const uint64_t CompositingReasonComboAllDirectReasons =
     CompositingReason3DTransform | CompositingReasonVideo |
     CompositingReasonCanvas | CompositingReasonPlugin |
     CompositingReasonIFrame | CompositingReasonBackfaceVisibilityHidden |
     CompositingReasonActiveAnimation | CompositingReasonTransitionProperty |
     CompositingReasonScrollDependentPosition |
     CompositingReasonOverflowScrollingTouch |
     CompositingReasonOverflowScrollingParent |
@@ skipping 70 matching lines @@
   const char* description;
 };
 
 PLATFORM_EXPORT extern const CompositingReasonStringMap
     kCompositingReasonStringMap[];
 PLATFORM_EXPORT extern const size_t kNumberOfCompositingReasons;
 
 }  // namespace blink
 
 #endif  // CompositingReasons_h