VR: Return the floor to being hit-testable

At some point, the floor regressed to be non-hittable.  Mark it as
explicitly hit-testable.

BUG=853822
R=tiborg

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:linux_vr;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: Ie8ceda269a3e403c4fbf3eeb46709344d1faa90f
Reviewed-on: https://chromium-review.googlesource.com/1104980Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568174}

chrome/browser/vr/ui_scene_creator.cc

@@ -903,6 +903,7 @@ std::unique_ptr<Grid> CreateGrid(Model* model, UiElementName name) {
   grid->SetRotate(1, 0, 0, -base::kPiFloat / 2);
   grid->set_gridline_count(kFloorGridlineCount);
   grid->SetGridColor(model->color_scheme().floor_grid);
+  grid->set_hit_testable(true);
   grid->set_focusable(false);
   return grid;
 }