After playing with the new behavior (the reticle stacking atop UI), I actually think it feels better (and would permit drawing the controller into a more power-efficient overlay). cc'ing gordonbrander to check this out.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6cd5b164f2d3f4882ee29657a7b880aed0bbdafb

commit 6cd5b164f2d3f4882ee29657a7b880aed0bbdafb
Author: Ian Vollick <vollick@chromium.org>
Date: Wed Nov 01 23:03:41 2017

[vr] Fix reticle sorting

With this change, UiScene inserts the reticle into the lists of elements
that it vends out to the UiRenderer, fixing a reticle rendering
regression.

Bug:  779692 
Change-Id: Ib62f50b4fefe3f0e8e05aa59edfbe1bc37a3104b
Reviewed-on: https://chromium-review.googlesource.com/749741
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#513309}
[modify] https://crrev.com/6cd5b164f2d3f4882ee29657a7b880aed0bbdafb/chrome/browser/vr/elements/reticle.cc
[modify] https://crrev.com/6cd5b164f2d3f4882ee29657a7b880aed0bbdafb/chrome/browser/vr/elements/reticle.h
[modify] https://crrev.com/6cd5b164f2d3f4882ee29657a7b880aed0bbdafb/chrome/browser/vr/ui_renderer_unittest.cc
[modify] https://crrev.com/6cd5b164f2d3f4882ee29657a7b880aed0bbdafb/chrome/browser/vr/ui_scene.cc

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/b3df3978ff3856299768d5a0afd3c7276c99c650

commit b3df3978ff3856299768d5a0afd3c7276c99c650
Author: Ian Vollick <vollick@chromium.org>
Date: Thu Dec 14 20:17:27 2017

[vr] Draw the reticle after all elements

This change reverts my previous attempt at sorting the reticle into the
list of UI elements. It turns out that this is more subtle than I'd
originally appreciated, and until we can come up with a robust approach
that avoids all artifacts, I'm reverting to a state where the reticle
consistently draws atop all UI elements.

Bug:  779692 
Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: I185b70f4c0137a6513c67765c1696fe16f996352
Reviewed-on: https://chromium-review.googlesource.com/826431
Reviewed-by: Michael Thiessen <mthiesse@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#524156}
[modify] https://crrev.com/b3df3978ff3856299768d5a0afd3c7276c99c650/chrome/browser/vr/elements/reticle.cc
[modify] https://crrev.com/b3df3978ff3856299768d5a0afd3c7276c99c650/chrome/browser/vr/ui_scene.cc
[modify] https://crrev.com/b3df3978ff3856299768d5a0afd3c7276c99c650/chrome/browser/vr/ui_unittest.cc

I think our tree-order sorting makes this obsolete.