bluetooth: readValue Blink interface
bluetooth: readValue Blink interface

Add the necessary interface for readValue

https://webbluetoothcg.github.io/web-bluetooth/#dom-bluetoothgattcharacteristic-readvalue

This is the firs of three patches to implement readValue:

  [1] This patch.
  [2] http://crrev.com/1149883011
  [3] http://crrev.com/1147243004


BUG=496379

Review URL: https://codereview.chromium.org/1168113008

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@196990 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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