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This CL ports the GATT API bindings from the D-Bus API in our Chrome OS fork to
the API definition in doc/gatt-api.txt in BlueZ upstream.

TBR=keybuk@chromium.org
BUG=440968

Review URL: https://codereview.chromium.org/788193004

Cr-Commit-Position: refs/heads/master@{#315095}