VR: Add back mistakenly deleted code.

Looks like some code that was commented as being for autopresentation
wasn't actually for autopresentation :)

Code was erroneously removed in
https://chromium-review.googlesource.com/c/chromium/src/+/1127850

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: Ieb871d92ed97f6df814d51f0a508f9b7d5b1456c
Reviewed-on: https://chromium-review.googlesource.com/1129855Reviewed-by: Brian Sheedy <bsheedy@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573444}

chrome/browser/vr/ui.cc

@@ -85,10 +85,12 @@ void Ui::SetWebVrMode(bool enabled) {
   if (enabled) {
     model_->web_vr.has_received_permissions = false;
     model_->web_vr.state = kWebVrAwaitingFirstFrame;
-    model_->push_mode(kModeWebVr);
+    if (!model_->web_vr_enabled())
+      model_->push_mode(kModeWebVr);
   } else {
     model_->web_vr.state = kWebVrNoTimeoutPending;
-    model_->pop_mode();
+    if (model_->web_vr_enabled())
+      model_->pop_mode();
   }
 }