Add counters to count the numbers of legacy and LayoutNG LayoutBlocks

This CL has no user-visible behavior changes.

Note that LayoutNG ratio computed by these counters is 83.1%
on nytiimes.com.

Bug: 1067527
Change-Id: I0f2628d3eb5c4d7da7a4d5b3cc44cc918137a17b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2138653Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756945}

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

@@ -680,6 +680,8 @@ class CORE_EXPORT Document : public ContainerNode,
   void UpdateStyleAndLayoutForNode(const Node*, DocumentUpdateReason);
   void IncLayoutCallsCounter() { ++layout_calls_counter_; }
   void IncLayoutCallsCounterNG() { ++layout_calls_counter_ng_; }
+  void IncLayoutBlockCounter() { ++layout_blocks_counter_; }
+  void IncLayoutBlockCounterNG() { ++layout_blocks_counter_ng_; }
 
   scoped_refptr<const ComputedStyle> StyleForPage(int page_index);
 
@@ -2218,6 +2220,13 @@ class CORE_EXPORT Document : public ContainerNode,
   // The number of LayoutObject::UpdateLayout() calls for LayoutNG.
   uint32_t layout_calls_counter_ng_ = 0;
 
+  // The number of LayoutBlock instances for both of the legacy layout
+  // and LayoutNG.
+  uint32_t layout_blocks_counter_ = 0;
+
+  // The number of LayoutNGMixin<LayoutBlock> instances
+  uint32_t layout_blocks_counter_ng_ = 0;
+
   bool deferred_compositor_commit_is_allowed_ = false;
 
   // True when the document was created (in DomImplementation) for specific MIME

third_party/blink/renderer/core/layout/layout_block.cc

@@ -111,6 +111,9 @@ LayoutBlock::LayoutBlock(ContainerNode* node)
       has_percent_height_descendants_(false),
       pagination_state_changed_(false),
       is_legacy_initiated_out_of_flow_layout_(false) {
+  if (node)
+    GetDocument().IncLayoutBlockCounter();
+
   // LayoutBlockFlow calls setChildrenInline(true).
   // By default, subclasses do not have inline children.
 }

third_party/blink/renderer/core/layout/ng/layout_ng_mixin.cc

@@ -26,6 +26,8 @@ LayoutNGMixin<Base>::LayoutNGMixin(Element* element) : Base(element) {
       std::is_base_of<LayoutBlock, Base>::value,
       "Base class of LayoutNGMixin must be LayoutBlock or derived class.");
   DCHECK(!element || !element->ShouldForceLegacyLayout());
+  if (element)
+    Base::GetDocument().IncLayoutBlockCounterNG();
 }
 
 template <typename Base>