[FCP++] Performance: avoid creating ID for unrelated nodes

FCP++ uses IdForNode() to get node ID. But IdForNode() has the side effect of
creating an id for node if it doesn't exist. As FCP++ uses IdForNode() for
every removed node regardless of whether it's content node, it's a costly
operation.

FCP++ only cares about the removal of the nodes that have already been
recorded. This indicates that the nodes already have node id. Based on
this characteristic, FCP++ can ignore the nodes that do not have node id, so
as to reduce cost.

This CL is to implement this idea.

Bug: 945405
Change-Id: I03602d3d79f5f5ed4c087a7a2cf0a44aec1accbb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1540263
Commit-Queue: Liquan (Max) Gu <maxlg@chromium.org>
Reviewed-by: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644869}

third_party/blink/renderer/core/dom/dom_node_ids.cc

@@ -10,6 +10,12 @@ namespace blink {
 
 DEFINE_WEAK_IDENTIFIER_MAP(Node, DOMNodeId)
 
+// static
+DOMNodeId DOMNodeIds::ExistingIdForNode(Node* node) {
+  return node ? WeakIdentifierMap<Node, DOMNodeId>::ExistingIdentifier(node)
+              : kInvalidDOMNodeId;
+}
+
 // static
 DOMNodeId DOMNodeIds::IdForNode(Node* node) {
   return node ? WeakIdentifierMap<Node, DOMNodeId>::Identifier(node)

third_party/blink/renderer/core/dom/dom_node_ids.h

@@ -19,6 +19,7 @@ class CORE_EXPORT DOMNodeIds {
   STATIC_ONLY(DOMNodeIds);
 
  public:
+  static DOMNodeId ExistingIdForNode(Node*);
   static DOMNodeId IdForNode(Node*);
   static Node* NodeForId(DOMNodeId);
 };

third_party/blink/renderer/core/dom/weak_identifier_map.h

@@ -38,6 +38,10 @@ class WeakIdentifierMap final
     return result;
   }
 
+  static IdentifierType ExistingIdentifier(T* object) {
+    return Instance().object_to_identifier_.at(object);
+  }
+
   static T* Lookup(IdentifierType identifier) {
     return Instance().identifier_to_object_.at(identifier);
   }

third_party/blink/renderer/core/paint/paint_timing_detector.cc

@@ -85,9 +85,9 @@ void PaintTimingDetector::NotifyTextPaint(
 }
 
 void PaintTimingDetector::NotifyNodeRemoved(const LayoutObject& object) {
-  if (!object.GetNode())
+  DOMNodeId node_id = DOMNodeIds::ExistingIdForNode(object.GetNode());
+  if (node_id == kInvalidDOMNodeId)
     return;
-  DOMNodeId node_id = DOMNodeIds::IdForNode(object.GetNode());
   text_paint_timing_detector_->NotifyNodeRemoved(node_id);
   image_paint_timing_detector_->NotifyNodeRemoved(node_id);
 }