Revert "cros: Enable underlay strategy"

This reverts commit 9e00c38.

Reason for revert: This seem to have caused at least 2 serious rendering artifacts:

crbug.com/854790
crbug.com/854677

Original change's description:
> cros: Enable underlay strategy
> 
> Add "underlay" to the list of overlay strategies on all the CrOS
> platform that are using drm atomic.
> 
> This strategy will make it possible to use HW overlays for
> content like video/webgl even when it's partially occluded by
> other HTML content.
> 
> Bug: 789288
> Change-Id: I834d129c7dcd359bfd00eed8df820a8d3b134ee2
> Reviewed-on: https://chromium-review.googlesource.com/1081307
> Reviewed-by: Kristian H. Kristensen <hoegsberg@chromium.org>
> Reviewed-by: Avi Drissman <avi@chromium.org>
> Commit-Queue: Daniele Castagna <dcastagna@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#563466}

TBR=avi@chromium.org,dcastagna@chromium.org,hoegsberg@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 789288, 854790, 854677
Change-Id: Ia46fe68d42ced9f625553120dd3c98096a820b99
Reviewed-on: https://chromium-review.googlesource.com/1110778
Reviewed-by: David Reveman <reveman@chromium.org>
Commit-Queue: David Reveman <reveman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569445}

content/browser/compositor/gpu_process_transport_factory.cc

@@ -266,7 +266,7 @@ CreateOverlayCandidateValidator(
       ozone_platform->GetOverlayManager();
   if (!command_line->HasSwitch(switches::kEnableHardwareOverlays) &&
       overlay_manager->SupportsOverlays()) {
-    enable_overlay_flag = "single-fullscreen,single-on-top,underlay";
+    enable_overlay_flag = "single-fullscreen,single-on-top";
   }
   if (!enable_overlay_flag.empty()) {
     std::unique_ptr<ui::OverlayCandidatesOzone> overlay_candidates =