Initial viewport is not the same as FrameView rect.

third_party/WebKit/Source/web/WebLocalFrameImpl.cpp

 /*
  * Copyright (C) 2009 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 102 matching lines @@
 #include "core/editing/PlainTextRange.h"
 #include "core/editing/TextAffinity.h"
 #include "core/editing/iterators/TextIterator.h"
 #include "core/editing/serializers/Serialization.h"
 #include "core/editing/spellcheck/SpellChecker.h"
 #include "core/fetch/ResourceFetcher.h"
 #include "core/fetch/SubstituteData.h"
 #include "core/frame/FrameHost.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalDOMWindow.h"
+#include "core/frame/PageScaleConstraintsSet.h"
 #include "core/frame/RemoteFrame.h"
 #include "core/frame/Settings.h"
 #include "core/frame/UseCounter.h"
 #include "core/frame/VisualViewport.h"
 #include "core/html/HTMLAnchorElement.h"
 #include "core/html/HTMLCollection.h"
 #include "core/html/HTMLFormElement.h"
 #include "core/html/HTMLFrameElementBase.h"
 #include "core/html/HTMLFrameOwnerElement.h"
 #include "core/html/HTMLHeadElement.h"
@@ skipping 1571 matching lines @@
   bool isMainFrame = !parent();
   IntSize initialSize = (isMainFrame || !frameWidget())
                             ? webView->mainFrameSize()
                             : (IntSize)frameWidget()->size();
   bool isTransparent = !isMainFrame && parent()->isWebRemoteFrame()
                            ? true
                            : webView->isTransparent();
 
   frame()->createView(initialSize, webView->baseBackgroundColor(),
                       isTransparent);
+  if (isMainFrame) {
+    frame()->view()->setInitialViewportSize(
+        webView->pageScaleConstraintsSet().initialViewportSize());
+  }
   if (webView->shouldAutoResize() && frame()->isLocalRoot())
     frame()->view()->enableAutoSizeMode(webView->minAutoSize(),
                                         webView->maxAutoSize());
 
   frame()->view()->setInputEventsTransformForEmulation(
       m_inputEventsOffsetForEmulation, m_inputEventsScaleFactorForEmulation);
   frame()->view()->setDisplayMode(webView->displayMode());
 }
 
 WebLocalFrameImpl* WebLocalFrameImpl::fromFrame(LocalFrame* frame) {
@@ skipping 657 matching lines @@
     feature = UseCounter::ChromeLoadTimesNpnNegotiatedProtocol;
   } else if (metric == "wasAlternateProtocolAvailable") {
     feature = UseCounter::ChromeLoadTimesWasAlternateProtocolAvailable;
   } else if (metric == "connectionInfo") {
     feature = UseCounter::ChromeLoadTimesConnectionInfo;
   }
   UseCounter::count(frame(), feature);
 }
 
 }  // namespace blink