Initial viewport is not the same as FrameView rect.

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

 /*
  * Copyright (C) 2013 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 89 matching lines @@
   // factor needs to be reset to its initial value.
   void setNeedsReset(bool);
   bool needsReset() const { return m_needsReset; }
 
   // This is set when one of the inputs to finalConstraints changes.
   bool constraintsDirty() const { return m_constraintsDirty; }
 
   void didChangeInitialContainingBlockSize(const IntSize&);
 
   IntSize layoutSize() const;
+  IntSize initialViewportSize() const { return m_icbSize; }
 
  private:
   PageScaleConstraintsSet();
 
   PageScaleConstraints computeConstraintsStack() const;
 
   PageScaleConstraints m_defaultConstraints;
   PageScaleConstraints m_pageDefinedConstraints;
   PageScaleConstraints m_userAgentConstraints;
   PageScaleConstraints m_fullscreenConstraints;
   PageScaleConstraints m_finalConstraints;
 
   int m_lastContentsWidth;
   IntSize m_icbSize;
 
   bool m_needsReset;
   bool m_constraintsDirty;
 };
 
 }  // namespace blink
 
 #endif  // PageScaleConstraintsSet_h