Don't cast WebWidget to WebFrameWidget if RenderWidget is frozen.

When frozen there is no WebFrameWidget, so this cast is invalid.

R=dcheng@chromium.org

Change-Id: I7a103aeeeb9920d128e1b8ad0b6075fc830ca0d6
Bug: 920952, 917907
Reviewed-on: https://chromium-review.googlesource.com/c/1407038Reviewed-by: Avi Drissman <avi@chromium.org>
Commit-Queue: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622080}

content/renderer/render_widget.cc

@@ -2182,10 +2182,11 @@ void RenderWidget::OnDragSourceEnded(const gfx::PointF& client_point,
 }
 
 void RenderWidget::OnDragSourceSystemDragEnded() {
-  if (!GetWebWidget())
+  blink::WebFrameWidget* frame_widget = GetFrameWidget();
+  if (!frame_widget)
     return;
 
-  static_cast<WebFrameWidget*>(GetWebWidget())->DragSourceSystemDragEnded();
+  frame_widget->DragSourceSystemDragEnded();
 }
 
 void RenderWidget::ShowVirtualKeyboardOnElementFocus() {