Fix crash with viewport units when using auto resize mode.

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 b9b322be5b1dbb6d6cfec3aee598b8d28b9381ab..0acb140fc89981e6b37d121bd0a78ad26222cc16 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -1379,9 +1379,9 @@ FloatSize FrameView::viewportSizeForViewportUnits() const {
   // 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);
   }