Initial viewport is not the same as FrameView rect.

third_party/WebKit/Source/core/css/MediaValuesInitialViewport.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/css/MediaValuesInitialViewport.h"
 
 #include "core/dom/Document.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
 #include "core/layout/api/LayoutViewItem.h"
 
 namespace blink {
 
 MediaValues* MediaValuesInitialViewport::create(LocalFrame& frame) {
   return new MediaValuesInitialViewport(frame);
 }
 
 MediaValuesInitialViewport::MediaValuesInitialViewport(LocalFrame& frame)
     : MediaValuesDynamic(&frame) {}
 
 double MediaValuesInitialViewport::viewportWidth() const {
-  DCHECK(m_frame->view() && m_frame->document());
-  int viewportWidth = m_frame->view()->frameRect().width();
-  return adjustDoubleForAbsoluteZoom(
-      viewportWidth, m_frame->document()->layoutViewItem().styleRef());
+  DCHECK(m_frame->view());
+  return m_frame->view()->initialViewportWidth();
 }
 
 double MediaValuesInitialViewport::viewportHeight() const {
-  DCHECK(m_frame->view() && m_frame->document());
-  int viewportHeight = m_frame->view()->frameRect().height();
-  return adjustDoubleForAbsoluteZoom(
-      viewportHeight, m_frame->document()->layoutViewItem().styleRef());
+  DCHECK(m_frame->view());
+  return m_frame->view()->initialViewportHeight();
 }
 
 }  // namespace blink