Add counters for LayoutObject::UpdateLayout

This CL has no user-visible behavior changes, but might have
performance impact.

Note that LayoutNG ratio computed by these counters is 83.7% on
nytimes.com.

Bug: 1067527
Change-Id: I73cda07e55e9c9a7d8cef580ebf59fda10b61d1e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2134031Reviewed-by: Yoshifumi Inoue <yosin@chromium.org>
Reviewed-by: Koji Ishii <kojii@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756201}

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

@@ -680,6 +680,9 @@ class CORE_EXPORT Document : public ContainerNode,
     kRunPostLayoutTasksSynchronously,
   };
   void UpdateStyleAndLayoutForNode(const Node*, DocumentUpdateReason);
+  void IncLayoutCallsCounter() { ++layout_calls_counter_; }
+  void IncLayoutCallsCounterNG() { ++layout_calls_counter_ng_; }
+
   scoped_refptr<const ComputedStyle> StyleForPage(int page_index);
 
   // Ensures that location-based data will be valid for a given node.
@@ -2253,6 +2256,13 @@ class CORE_EXPORT Document : public ContainerNode,
   // The number of canvas elements on the document
   int num_canvases_ = 0;
 
+  // The number of LayoutObject::UpdateLayout() calls for both of the legacy
+  // layout and LayoutNG.
+  uint32_t layout_calls_counter_ = 0;
+
+  // The number of LayoutObject::UpdateLayout() calls for LayoutNG.
+  uint32_t layout_calls_counter_ng_ = 0;
+
   // Number of display locks in this document that block all activation.
   int display_lock_blocking_all_activation_count_ = 0;
   // Number of locked display locks in the document.

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

@@ -886,6 +886,11 @@ void LayoutBox::UpdateAfterLayout() {
   // Legacy, then NG again, NG won't use a stale layout result.
   if (IsOutOfFlowPositioned() && !IsLayoutNGObject())
     ClearLayoutResults();
+
+  Document& document = GetDocument();
+  document.IncLayoutCallsCounter();
+  if (IsLayoutNGObject())
+    document.IncLayoutCallsCounterNG();
 }
 
 bool LayoutBox::HasOverrideIntrinsicContentWidth() const {