Initial viewport is not the same as FrameView rect.

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 d8f183533b304cbc72c827789d3e7ae11255013c..31141658c7bf88be093cb902c9d491e191359cb0 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.h
+++ b/third_party/WebKit/Source/core/frame/FrameView.h
@@ -165,6 +165,10 @@ class CORE_EXPORT FrameView final
   }
   bool layoutSizeFixedToFrameSize() { return m_layoutSizeFixedToFrameSize; }
 
+  void setInitialViewportSize(const IntSize&);
+  int initialViewportWidth() const { return m_initialViewportSize.width(); }
+  int initialViewportHeight() const { return m_initialViewportSize.height(); }
+
   void updateAcceleratedCompositingSettings();
 
   void recalcOverflowAfterStyleChange();
@@ -977,6 +981,7 @@ class CORE_EXPORT FrameView final
   float m_inputEventsScaleFactorForEmulation;
 
   IntSize m_layoutSize;
+  IntSize m_initialViewportSize;
   bool m_layoutSizeFixedToFrameSize;
 
   Timer<FrameView> m_didScrollTimer;