Add CHECKs to confirm exactly what is null when segfault occurs.

It's almost certainly the view but let's be sure!

Bug: 1139104
Change-Id: I8b12739be19d286602ee5770354345bd377c28f1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500964
Auto-Submit: Fergal Daly <fergal@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#821120}

third_party/blink/renderer/core/frame/web_view_frame_widget.cc

@@ -110,6 +110,11 @@ void WebViewFrameWidget::EndCommitCompositorFrame(
       commit_start_time);
   web_view_->UpdatePreferredSize();
 
+  // TODO(https://crbug.com/1139104): Remove these CHECKS.
+  CHECK(web_view_);
+  CHECK(web_view_->MainFrameImpl());
+  CHECK(web_view_->MainFrameImpl()->GetFrame());
+  CHECK(web_view_->MainFrameImpl()->GetFrame()->View());
   web_view_->MainFrameImpl()
       ->GetFrame()
       ->View()