The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/d65f1e79a2d0f48d998bf732d341f9112f9aff83

commit d65f1e79a2d0f48d998bf732d341f9112f9aff83
Author: Christopher Grant <cjgrant@chromium.org>
Date: Sat Mar 24 00:14:53 2018

VR: Make sounds consistent across all buttons

Have the button class itself assign default sounds.  This ensures they
all sound the same, in both enabled and disabled states.

BUG= 823413 

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;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Id254c67f702d7cd7ccd169b55f68f9b79cf4eb49
Reviewed-on: https://chromium-review.googlesource.com/978631
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545645}
[modify] https://crrev.com/d65f1e79a2d0f48d998bf732d341f9112f9aff83/chrome/browser/vr/elements/button.cc
[modify] https://crrev.com/d65f1e79a2d0f48d998bf732d341f9112f9aff83/chrome/browser/vr/elements/button.h
[modify] https://crrev.com/d65f1e79a2d0f48d998bf732d341f9112f9aff83/chrome/browser/vr/elements/button_unittest.cc
[modify] https://crrev.com/d65f1e79a2d0f48d998bf732d341f9112f9aff83/chrome/browser/vr/elements/disc_button.cc
[modify] https://crrev.com/d65f1e79a2d0f48d998bf732d341f9112f9aff83/chrome/browser/vr/elements/ui_element.cc
[modify] https://crrev.com/d65f1e79a2d0f48d998bf732d341f9112f9aff83/chrome/browser/vr/elements/ui_element.h
[modify] https://crrev.com/d65f1e79a2d0f48d998bf732d341f9112f9aff83/chrome/browser/vr/elements/vector_icon_button.cc
[modify] https://crrev.com/d65f1e79a2d0f48d998bf732d341f9112f9aff83/chrome/browser/vr/ui_scene_creator.cc

For verification:  The disabled sound should apply on the navigate-back button, when there is no page to go back to, and the navigate-forward button in the overflow menu, before the user has navigated back (ie. the button is disabled).