Initial viewport is not the same as FrameView rect.

third_party/WebKit/Source/web/WebLocalFrameImpl.cpp

Index: third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
diff --git a/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp b/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
index efe1fe56aa7c0a9d1a22a1ee8207305873fa102a..b69fa59322144253bfe712c0f589afbdcdd9a763 100644
--- a/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
+++ b/third_party/WebKit/Source/web/WebLocalFrameImpl.cpp
@@ -120,6 +120,7 @@
 #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"
@@ -1711,6 +1712,10 @@ void WebLocalFrameImpl::createFrameView() {
 
   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());