Add a DCHECK that render widget exists for auto-resize.

Bug: 912193
Change-Id: I3fd71adb55339c536d228e0b7a20fa64fbb6dd3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1829588
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Auto-Submit: Erik Chen <erikchen@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#701520}

content/renderer/render_view_impl.cc

@@ -2132,6 +2132,8 @@ void RenderViewImpl::PageImportanceSignalsChanged() {
 }
 
 void RenderViewImpl::DidAutoResize(const blink::WebSize& newSize) {
+  // Auto resize should only happen on local main frames.
+  DCHECK(render_widget_);
   render_widget_->DidAutoResize(newSize);
 }