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With this change, I've added a hover offset to the mic button. I've also
generalized some button code so that it can be reused by the mic button.
(Specifically, you can now set the corner radius of the disc button, and
we may want to rename this class eventually).

Bug: 818008
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: I0884b90ae19d34952c639653850ede4754e16fc8
Reviewed-on: https://chromium-review.googlesource.com/958221Reviewed-by: Biao She <bshe@chromium.org>
Commit-Queue: Ian Vollick <vollick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#542536}