Add a CHECK to ensure that widget_base is destroyed correctly.

I suspect something odd is going on with RenderDocument not marking the
Frame as detached. Marking the frame as detached Closes the
WebFrameWidgetBase via RenderWidget::CloseForFrame.

Turn this into a strong assert (there already is a DCHECK in
RenderWidget) that asserts the same thing.

BUG=1139104

Change-Id: If164f299a2fa9964747556dee38216f7ff78a799
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2487705
Commit-Queue: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: danakj <danakj@chromium.org>
Cr-Commit-Position: refs/heads/master@{#819191}

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

@@ -183,7 +183,13 @@ WebFrameWidgetBase::WebFrameWidgetBase(
                  ThreadScheduler::Current()->DeprecatedDefaultTaskRunner());
 }
 
-WebFrameWidgetBase::~WebFrameWidgetBase() = default;
+WebFrameWidgetBase::~WebFrameWidgetBase() {
+  // Ensure that Close is called and we aren't releasing |widget_base_| in the
+  // destructor.
+  // TODO(crbug.com/1139104): This CHECK can be changed to a DCHECK once
+  // the issue is solved.
+  CHECK(!widget_base_);
+}
 
 void WebFrameWidgetBase::BindLocalRoot(WebLocalFrame& local_root) {
   local_root_ = To<WebLocalFrameImpl>(local_root);