Renderer.LineLayoutMs UMA histogram to measure line layout time.

Source/core/frame/FrameView.h

 /*
    Copyright (C) 1997 Martin Jones (mjones@kde.org)
              (C) 1998 Waldo Bastian (bastian@kde.org)
              (C) 1998, 1999 Torben Weis (weis@kde.org)
              (C) 1999 Lars Knoll (knoll@kde.org)
              (C) 1999 Antti Koivisto (koivisto@kde.org)
    Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009 Apple Inc. All rights reserved.
 
    This library is free software; you can redistribute it and/or
    modify it under the terms of the GNU Library General Public
@@ skipping 533 matching lines @@
 
     virtual void trace(Visitor*) override;
     void notifyPageThatContentAreaWillPaint() const;
     FrameView* parentFrameView() const;
 
     void calculateOverhangAreasForPainting(IntRect& horizontalOverhangRect, IntRect& verticalOverhangRect);
 
     // Returns the scrollable area for the frame.
     ScrollableArea* scrollableArea();
 
+    void lineLayoutTime(double ms);
+
 protected:
     // Scroll the content via the compositor.
     bool scrollContentsFastPath(const IntSize& scrollDelta);
 
     // Scroll the content by invalidating everything.
     void scrollContentsSlowPath(const IntRect& updateRect);
 
     bool isVerticalDocument() const;
     bool isFlippedDocument() const;
 
@@ skipping 253 matching lines @@
     bool m_scrollbarsSuppressed;
 
     FloatSize m_elasticOverscroll;
 
     bool m_inUpdateScrollbars;
 
     IntPoint m_panScrollIconPoint;
     bool m_shouldDrawPanScrollIcon;
 
     bool m_clipsRepaints;
+
+    double m_lineLayoutMs;
 };
 
 inline void FrameView::incrementVisuallyNonEmptyCharacterCount(unsigned count)
 {
     if (m_isVisuallyNonEmpty)
         return;
     m_visuallyNonEmptyCharacterCount += count;
     // Use a threshold value to prevent very small amounts of visible content from triggering didFirstVisuallyNonEmptyLayout.
     // The first few hundred characters rarely contain the interesting content of the page.
     static const unsigned visualCharacterThreshold = 200;
@@ skipping 36 matching lines @@
     }
 
 private:
     RawPtrWillBeMember<FrameView> m_view;
     bool m_originalValue;
 };
 
 } // namespace blink
 
 #endif // FrameView_h