Initial viewport is not the same as FrameView rect.

third_party/WebKit/Source/core/css/MediaValuesInitialViewportTest.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/frame/FrameView.h"
 #include "core/testing/DummyPageHolder.h"
 #include "testing/gtest/include/gtest/gtest.h"
 
 namespace blink {
 
 class MediaValuesInitialViewportTest : public ::testing::Test {
  protected:
   Document& document() const { return m_dummyPageHolder->document(); }
 
  private:
   void SetUp() override {
     m_dummyPageHolder = DummyPageHolder::create(IntSize(320, 480));
+    document().view()->setInitialViewportSize(IntSize(320, 480));
   }
 
   std::unique_ptr<DummyPageHolder> m_dummyPageHolder;
 };
 
 TEST_F(MediaValuesInitialViewportTest, InitialViewportSize) {
   FrameView* view = document().view();
   ASSERT_TRUE(view);
   EXPECT_TRUE(view->layoutSizeFixedToFrameSize());
 
   MediaValues* mediaValues =
       MediaValuesInitialViewport::create(*document().frame());
   EXPECT_EQ(320, mediaValues->viewportWidth());
   EXPECT_EQ(480, mediaValues->viewportHeight());
 
   view->setLayoutSizeFixedToFrameSize(false);
   view->setLayoutSize(IntSize(800, 600));
   EXPECT_EQ(320, mediaValues->viewportWidth());
   EXPECT_EQ(480, mediaValues->viewportHeight());
 }
 
 }  // namespace blink