Initial viewport is not the same as FrameView rect.

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

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 4462 matching lines @@
   return canThrottleRendering() && lifecycle().throttlingAllowed();
 }
 
 bool FrameView::canThrottleRendering() const {
   if (!RuntimeEnabledFeatures::renderingPipelineThrottlingEnabled())
     return false;
   return m_subtreeThrottled ||
          (m_hiddenForThrottling && m_crossOriginForThrottling);
 }
 
+void FrameView::setInitialViewportSize(const IntSize& viewportSize) {
+  if (viewportSize == m_initialViewportSize)
+    return;
+
+  m_initialViewportSize = viewportSize;
+  // TODO(rune@opera.com): Notify the viewport style resolver about the changed
+  // initial viewport:
+  //
+  // if (Document* document = m_frame->document())
+  //   document->styleEngine().initialViewportChanged();
+}
+
+int FrameView::initialViewportWidth() const {
+  DCHECK(m_frame->isMainFrame());
+  return m_initialViewportSize.width();
+}
+
+int FrameView::initialViewportHeight() const {
+  DCHECK(m_frame->isMainFrame());
+  return m_initialViewportSize.height();
+}
+
 }  // namespace blink