bluetooth: BluetoothDiscovery::requestDevice uses Platform::bluetooth().

In three patches the initial mock implementation of bluetooth.requestDevice
is implemented across blink & content. This allows layout tests to specify the
data responses the mock implemented in content should return via
testRunner.SetBluetoothMockDataSet and call bluetooth.requestDevice with the
expected results.

crrev.com/650613005 blink::WebBluetooth & WebBluetoothError interfaces.
crrev.com/702593002 content::WebBluetoothImpl & testRunner.SetBluetoothMockDataSet.
crrev.com/686813003 blink::BluetoothDiscovery::requestDevice implemented.

BUG=420284

Review URL: https://codereview.chromium.org/686813003

git-svn-id: svn://svn.chromium.org/blink/trunk@184926 bbb929c8-8fbe-4397-9dbb-9b2b20218538