[compositor-worker] root scrolling layer is associated with document scrolling element

third_party/WebKit/Source/platform/graphics/CompositorMutableStateProvider.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 "platform/graphics/CompositorMutableStateProvider.h"
 
 #include "cc/layers/layer_impl.h"
 #include "cc/trees/layer_tree_impl.h"
 #include "platform/graphics/CompositorElementId.h"
 #include "platform/graphics/CompositorMutableProperties.h"
 #include "platform/graphics/CompositorMutableState.h"
 #include "platform/graphics/CompositorMutation.h"
 #include "wtf/PtrUtil.h"
 #include <memory>
 
 namespace blink {
 
 CompositorMutableStateProvider::CompositorMutableStateProvider(
     cc::LayerTreeImpl* treeImpl,
     CompositorMutations* mutations)
     : m_tree(treeImpl), m_mutations(mutations) {}
 
 CompositorMutableStateProvider::~CompositorMutableStateProvider() {}
 
 std::unique_ptr<CompositorMutableState>
-CompositorMutableStateProvider::getMutableStateFor(uint64_t elementId) {
+CompositorMutableStateProvider::getMutableStateFor(
+    uint64_t elementId,
+    uint64_t scrollingElementId) {
   cc::LayerImpl* mainLayer = m_tree->LayerByElementId(
       createCompositorElementId(elementId, CompositorSubElementId::Primary));
-  cc::LayerImpl* scrollLayer = m_tree->LayerByElementId(
-      createCompositorElementId(elementId, CompositorSubElementId::Scroll));
+  cc::LayerImpl* scrollLayer =
+      m_tree->LayerByElementId(createCompositorElementId(
+          scrollingElementId, CompositorSubElementId::Scroll));
 
   if (!mainLayer && !scrollLayer)
     return nullptr;
 
   // Ensure that we have an entry in the map for |elementId| but do as few
   // allocations and queries as possible. This will update the map only if we
   // have not added a value for |elementId|.
   auto result = m_mutations->map.insert(elementId, nullptr);
 
   // Only if this is a new entry do we want to allocate a new mutation.
   if (result.isNewEntry)
     result.storedValue->value = WTF::wrapUnique(new CompositorMutation);
 
   return WTF::wrapUnique(new CompositorMutableState(
       result.storedValue->value.get(), mainLayer, scrollLayer));
 }
 
 }  // namespace blink