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Now connectGATT connects to the BLE device. This is the first
step towards fully implementing connectGATT.

Also adds BluetoothMsg_ConnectGATTError.

Layout tests are in:

http://crrev.com/1150523004

BUG=421668

Review URL: https://codereview.chromium.org/1120373004

Cr-Commit-Position: refs/heads/master@{#330883}