[Layout API] Convert ViewportScrollCallback to use Document::layoutViewItem()

There were several references to Document::layoutView()
which is deprecated by the new layout API. I converted them to use
layoutViewItem() instead. No new API methods were required.

There are no functional changes.

BUG=499321

Review URL: https://codereview.chromium.org/1923533003

Cr-Commit-Position: refs/heads/master@{#389911}

third_party/WebKit/Source/core/page/scrolling/ViewportScrollCallback.cpp

@@ -11,7 +11,7 @@
 #include "core/frame/TopControls.h"
 #include "core/frame/VisualViewport.h"
 #include "core/input/EventHandler.h"
-#include "core/layout/LayoutView.h"
+#include "core/layout/api/LayoutViewItem.h"
 #include "core/page/scrolling/ScrollState.h"
 #include "platform/geometry/FloatSize.h"
 #include "platform/scroll/ScrollableArea.h"
@@ -102,10 +102,10 @@ void ViewportScrollCallback::handleEvent(ScrollState* state)
 
 ScrollableArea* ViewportScrollCallback::getRootFrameViewport() const
 {
-    if (!m_document->layoutView())
+    if (m_document->layoutViewItem().isNull())
         return nullptr;
 
-    FrameView* frameView = m_document->layoutView()->frameView();
+    FrameView* frameView = m_document->layoutViewItem().frameView();
     if (!frameView)
         return nullptr;