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This change wires up button hover and click sounds, and adds them to the
asset structure.

BUG=812026
R=tiborg

Cq-Include-Trybots: master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_vr;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel
Change-Id: Iccf113e74663201c62aa220c7e134990369fd755
Reviewed-on: https://chromium-review.googlesource.com/953962
Commit-Queue: Christopher Grant <cjgrant@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Reviewed-by: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542109}