content: Don't clear background page if rendering timeout is disabled

The behaviour changed in crrev.com/c/995915 and some headless tests
are failing as a result.

Bug: 815187
Change-Id: Iff689a1a8ef0d0b464cf13701034bcca94ef0cf2
Reviewed-on: https://chromium-review.googlesource.com/1014015Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Saman Sami <samans@chromium.org>
Cr-Commit-Position: refs/heads/master@{#551100}

content/browser/renderer_host/render_widget_host_impl.cc

@@ -3140,10 +3140,11 @@ void RenderWidgetHostImpl::DidReceiveFirstFrameAfterNavigation() {
 }
 
 void RenderWidgetHostImpl::ForceFirstFrameAfterNavigationTimeout() {
-  if (did_receive_first_frame_after_navigation_)
+  if (did_receive_first_frame_after_navigation_ ||
+      !new_content_rendering_timeout_) {
     return;
-  if (new_content_rendering_timeout_)
-    new_content_rendering_timeout_->Stop();
+  }
+  new_content_rendering_timeout_->Stop();
   ClearDisplayedGraphics();
 }