Add DCHECK to RWHVAndroid::DestroyDelegatedContent

I removed a DCHECK by mistake in crrev.com/c/995915. Also add a
comment.

Bug: 815187
Change-Id: I5bffc2b23358a56ebf3a35621c6c9f2970519b58
Reviewed-on: https://chromium-review.googlesource.com/1005785Reviewed-by: Khushal <khushalsagar@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Saman Sami <samans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549690}

content/browser/renderer_host/render_widget_host_view_android.cc

@@ -977,6 +977,9 @@ void RenderWidgetHostViewAndroid::DestroyDelegatedContent() {
   if (!delegated_frame_host_)
     return;
 
+  DCHECK(delegated_frame_host_->HasDelegatedContent() ==
+         frame_evictor_->HasFrame());
+
   delegated_frame_host_->DestroyDelegatedContent();
 
   if (frame_evictor_->HasFrame())

ui/android/delegated_frame_host_android.cc

@@ -160,6 +160,8 @@ bool DelegatedFrameHostAndroid::CanCopyFromCompositingSurface() const {
 }
 
 void DelegatedFrameHostAndroid::DestroyDelegatedContent() {
+  // TakeFallbackContentFrom() can populate |content_layer_| when
+  // |surface_info_| is invalid.
   if (content_layer_) {
     content_layer_->RemoveFromParent();
     content_layer_ = nullptr;