Add compositor element id for scroll nodes in SPv2.

third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp

Index: third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp
diff --git a/third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp b/third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp
index 3a58fa717bfd01cd5d48348c23141404cd32f360..785aab858f8e54f2d258f7cc92936a6e30a8a128 100644
--- a/third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp
+++ b/third_party/WebKit/Source/platform/graphics/compositing/PropertyTreeManager.cpp
@@ -134,6 +134,7 @@ void PropertyTreeManager::setupRootScrollNode() {
   cc::ScrollTree& scrollTree = m_propertyTrees.scroll_tree;
   scrollTree.clear();
   m_propertyTrees.layer_id_to_scroll_node_index.clear();
+  m_propertyTrees.element_id_to_scroll_node_index.clear();

[wkorman, 2017/01/19 23:43:10]

This (and below) could be done in a separate change but I figured I'd throw it
in here as well since pdr@ will need it in short order. I have a separate change
where I'm doing this (and setting in field and map as below) for
transform/effect nodes.

   cc::ScrollNode& scrollNode =
       *scrollTree.Node(scrollTree.Insert(cc::ScrollNode(), kRealRootNodeId));
   DCHECK_EQ(scrollNode.id, kSecondaryRootNodeId);
@@ -267,6 +268,9 @@ int PropertyTreeManager::ensureCompositorScrollNode(
       ensureCompositorTransformNode(scrollNode->scrollOffsetTranslation());
   compositorNode.main_thread_scrolling_reasons =
       scrollNode->mainThreadScrollingReasons();
+  compositorNode.element_id = scrollNode->compositorElementId();

[wkorman, 2017/01/20 01:57:34]

Probably should wrap this in a block to make sure compositor element id exists
as I did in http://crrev.com/2642183003

+  m_propertyTrees.element_id_to_scroll_node_index[compositorNode.element_id] =
+      id;
 
   auto result = m_scrollNodeMap.set(scrollNode, id);
   DCHECK(result.isNewEntry);