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This change updates the previous, GATT-based code to use the new
authenticator::Platform class.

BUG=1002262

Binary-Size: This code ends up in a dynamic feature module, and so doesn't impact the main APK size. Also, the previous change (https://chromium-review.googlesource.com/c/chromium/src/+/2430324) deleted 13KiB of code so the delta isn't as large as it seems.
Change-Id: I5ed395492ac440d7fdab6a13edfef7610da505f8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2436415
Commit-Queue: Adam Langley <agl@chromium.org>
Reviewed-by: Martin Kreichgauer <martinkr@google.com>
Cr-Commit-Position: refs/heads/master@{#812408}