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

content/renderer/bluetooth/web_bluetooth_impl.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.
 
 #ifndef CONTENT_CHILD_BLUETOOTH_WEB_BLUETOOTH_IMPL_H_
 #define CONTENT_CHILD_BLUETOOTH_WEB_BLUETOOTH_IMPL_H_
 
 #include <string>
 
 #include "base/compiler_specific.h"
 #include "base/memory/ref_counted.h"
 #include "content/common/content_export.h"
 #include "third_party/WebKit/public/platform/modules/bluetooth/WebBluetooth.h"
 
 namespace content {
 
 class BluetoothDispatcher;
 class ThreadSafeSender;
 
 // Implementation of blink::WebBluetooth. Passes calls through to the thread
 // specific BluetoothDispatcher.
 class CONTENT_EXPORT WebBluetoothImpl
     : NON_EXPORTED_BASE(public blink::WebBluetooth) {
  public:
   explicit WebBluetoothImpl(ThreadSafeSender* thread_safe_sender);
+  WebBluetoothImpl(ThreadSafeSender* thread_safe_sender, int frame_routing_id);
   ~WebBluetoothImpl();
 
   // blink::WebBluetooth interface:
   void requestDevice(
       const blink::WebRequestDeviceOptions& options,
       blink::WebBluetoothRequestDeviceCallbacks* callbacks) override;
   void connectGATT(const blink::WebString& device_instance_id,
       blink::WebBluetoothConnectGATTCallbacks* callbacks) override;
   void getPrimaryService(
       const blink::WebString& device_instance_id,
       const blink::WebString& service_uuid,
       blink::WebBluetoothGetPrimaryServiceCallbacks* callbacks) override;
   void getCharacteristic(
       const blink::WebString& service_instance_id,
       const blink::WebString& characteristic_uuid,
       blink::WebBluetoothGetCharacteristicCallbacks* callbacks) override;
   void readValue(const blink::WebString& characteristic_instance_id,
                  blink::WebBluetoothReadValueCallbacks* callbacks) override;
   void writeValue(const blink::WebString& characteristic_instance_id,
                   const std::vector<uint8_t>& value,
                   blink::WebBluetoothWriteValueCallbacks*) override;
 
  private:
   BluetoothDispatcher* GetDispatcher();
 
-  scoped_refptr<ThreadSafeSender> thread_safe_sender_;
+  const scoped_refptr<ThreadSafeSender> thread_safe_sender_;
+  const int frame_routing_id_;
 
   DISALLOW_COPY_AND_ASSIGN(WebBluetoothImpl);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_CHILD_BLUETOOTH_WEB_BLUETOOTH_IMPL_H_