Remove background_color_ in DelegatedFrameHost

It's not used.

Change-Id: If99b1617dfd3fe83d87665c3d428d3348ad30d3b
Reviewed-on: https://chromium-review.googlesource.com/c/1301874Reviewed-by: Fady Samuel <fsamuel@chromium.org>
Commit-Queue: Fady Samuel <fsamuel@chromium.org>
Cr-Commit-Position: refs/heads/master@{#603119}

content/browser/renderer_host/delegated_frame_host.h

@@ -208,9 +208,6 @@ class CONTENT_EXPORT DelegatedFrameHost
   // TODO(ccameron): The meaning of "current" should be made more clear here.
   gfx::Size current_frame_size_in_dip_;
 
-  // This is the last root background color from a swapped frame.
-  SkColor background_color_;
-
   viz::HostFrameSinkManager* const host_frame_sink_manager_;
 
   // State for rendering into a Surface.