Fix crash with viewport units when using auto resize mode.

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

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 1361 matching lines @@
   FloatSize size(layoutSize(IncludeScrollbars));
 
   // We use the layoutSize rather than frameRect to calculate viewport units
   // so that we get correct results on mobile where the page is laid out into
   // a rect that may be larger than the viewport (e.g. the 980px fallback
   // width for desktop pages). Since the layout height is statically set to
   // be the viewport with browser controls showing, we add the browser controls
   // height, compensating for page scale as well, since we want to use the
   // viewport with browser controls hidden for vh (to match Safari).
   BrowserControls& browserControls = m_frame->host()->browserControls();
-  if (m_frame->isMainFrame() && size.width()) {
-    float pageScaleAtLayoutWidth =
-        m_frame->host()->visualViewport().size().width() / size.width();
+  int viewportWidth = m_frame->host()->visualViewport().size().width();
+  if (m_frame->isMainFrame() && size.width() && viewportWidth) {
+    float pageScaleAtLayoutWidth = viewportWidth / size.width();
     size.expand(0, browserControls.height() / pageScaleAtLayoutWidth);
   }
 
   size.scale(1 / zoom);
   return size;
 }
 
 DocumentLifecycle& FrameView::lifecycle() const {
   DCHECK(m_frame);
   DCHECK(m_frame->document());
@@ skipping 3521 matching lines @@
        reason < MainThreadScrollingReason::kMainThreadScrollingReasonCount;
        ++reason) {
     if (m_mainThreadScrollingReasonsCounter[reason] > 0) {
       reasons |= 1 << (reason - 1);
     }
   }
   return reasons;
 }
 
 }  // namespace blink