[vr] Re-enable the pruned tree walks

The issues now seem to be resolved.

Bug: 832650
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr
Change-Id: Id43bc440805fb4ed74d430ed7384eed78bce289f
Reviewed-on: https://chromium-review.googlesource.com/1011678Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#550627}

chrome/browser/vr/elements/ui_element.cc

@@ -23,7 +23,7 @@ namespace vr {
 
 namespace {
 
-constexpr bool kEnableOptimizedTreeWalks = false;
+constexpr bool kEnableOptimizedTreeWalks = true;
 constexpr float kHitTestResolutionInMeter = 0.000001f;
 
 int AllocateId() {