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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}