Put the RenderFrame's ID into a per-frame WebBluetooth, and plumb that back to the browser.

content/common/bluetooth/bluetooth_messages.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // Messages for Web Bluetooth API.
 // Multiply-included message file, hence no include guard.
 
 // Web Bluetooth Security
 // The security mechanisms of Bluetooth are described in the specification:
 // https://webbluetoothchrome.github.io/web-bluetooth
@@ skipping 168 matching lines @@
 // Informs the renderer that an error occurred while writing a value to a
 // characteristic.
 IPC_MESSAGE_CONTROL3(BluetoothMsg_WriteCharacteristicValueError,
                      int /* thread_id */,
                      int /* request_id */,
                      blink::WebBluetoothError /* result */)
 
 // Messages sent from the renderer to the browser.
 
 // Requests a bluetooth device from the browser.
-// TODO(scheib): UI to select and permit access to a device crbug.com/436280.
-//   This will include refactoring messages to be associated with an origin
-//   and making this initial requestDevice call with an associated frame.
-//   This work is deferred to simplify initial prototype patches.
-//   The Bluetooth feature, and the BluetoothDispatcherHost are behind
-//   the --enable-experimental-web-platform-features flag.
-IPC_MESSAGE_CONTROL4(BluetoothHostMsg_RequestDevice,
+IPC_MESSAGE_CONTROL5(BluetoothHostMsg_RequestDevice,
                      int /* thread_id */,
                      int /* request_id */,
+                     int /* routing_id */,

[scheib, 2015/07/16 01:11:46]

Yes, adding frame_routing_id here would make this clearer.

[Jeffrey Yasskin, 2015/07/17 01:32:44]

Done.

                      std::vector<content::BluetoothScanFilter>,
                      std::vector<device::BluetoothUUID> /* optional_services */)
 
 // Connects to a bluetooth device.
 IPC_MESSAGE_CONTROL3(BluetoothHostMsg_ConnectGATT,
                      int /* thread_id */,
                      int /* request_id */,
                      std::string /* device_instance_id */)
 
 // Gets primary service from bluetooth device.
@@ skipping 15 matching lines @@
                      int /* thread_id */,
                      int /* request_id */,
                      std::string /* characteristic_instance_id */)
 
 // Writes a value to a bluetooth device's characteristic.
 IPC_MESSAGE_CONTROL4(BluetoothHostMsg_WriteValue,
                      int /* thread_id */,
                      int /* request_id */,
                      std::string /* characteristic_instance_id */,
                      std::vector<uint8_t> /* value */)