Viewport resize anchoring should use the current layout viewport.

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

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index 50137729160c42130d6a2ec3d239691f308eca76..5068c0edcd802112adf2216c77a03f1645863b31 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -819,8 +819,13 @@ void FrameView::performPreLayoutTasks()
         m_viewportScrollableArea = RootFrameViewport::create(visualViewport, *layoutViewport);
     }
 
-    if (!m_scrollAnchor.hasScroller())
-        m_scrollAnchor.setScroller(m_viewportScrollableArea ? m_viewportScrollableArea : this);
+    if (!m_scrollAnchor.hasScroller()) {
+        ScrollableArea* scroller = m_viewportScrollableArea;
+        if (!scroller)
+            scroller = this;
+
+        m_scrollAnchor.setScroller(scroller);
+    }
 
     if (shouldPerformScrollAnchoring())
         m_scrollAnchor.save();
@@ -4131,6 +4136,11 @@ ScrollableArea* FrameView::layoutViewportScrollableArea()
     return layoutViewItem.isNull() ? nullptr : layoutViewItem.getScrollableArea();
 }
 
+RootFrameViewport* FrameView::getRootFrameViewport()
+{
+    return m_viewportScrollableArea.get();
+}
+
 LayoutObject* FrameView::viewportLayoutObject() const
 {
     if (Document* document = frame().document()) {