Adds DevTools commands for forced viewport override.

third_party/WebKit/Source/core/frame/FrameView.h

Index: third_party/WebKit/Source/core/frame/FrameView.h
diff --git a/third_party/WebKit/Source/core/frame/FrameView.h b/third_party/WebKit/Source/core/frame/FrameView.h
index 8c49dc36344679d0e98243295936aad2357daca9..f67aec418e044f3ca0e9dd86acc4f33f375c5a8f 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.h
+++ b/third_party/WebKit/Source/core/frame/FrameView.h
@@ -191,6 +191,7 @@ public:
     void setInputEventsTransformForEmulation(const IntSize&, float);
 
     void setScrollPosition(const DoublePoint&, ScrollType, ScrollBehavior = ScrollBehaviorInstant) override;
+    void didChangeScrollOffset();
 
     void didUpdateElasticOverscroll();
 
@@ -450,6 +451,14 @@ public:
     IntRect visibleContentRect(IncludeScrollbarsInRect = ExcludeScrollbars) const override;
     IntSize visibleContentSize(IncludeScrollbarsInRect = ExcludeScrollbars) const;
 
+    // Set a custom painting area. This is and should only be used for DevTools'
+    // visual transform overrides to ensure that the content inside the
+    // overridden visible area is painted. If not set, the painting area
+    // defaults to visibleContentRect().
+    void setVisibleContentRectForPainting(const IntRect&);
+    void resetVisibleContentRectForPainting();
+    IntRect visibleContentRectForPainting() const;
+
     // Functions for getting/setting the size of the document contained inside the FrameView (as an IntSize or as individual width and height
     // values).
     IntSize contentsSize() const override; // Always at least as big as the visibleWidth()/visibleHeight().
@@ -878,6 +887,7 @@ private:
     DoubleSize m_pendingScrollDelta;
     DoublePoint m_scrollPosition;
     IntSize m_contentsSize;
+    WTF::Optional<IntRect> m_visibleContentRectForPainting;
 
     int m_scrollbarsAvoidingResizer;
     bool m_scrollbarsSuppressed;