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

content/renderer/bluetooth/bluetooth_dispatcher.cc

Index: content/renderer/bluetooth/bluetooth_dispatcher.cc
diff --git a/content/renderer/bluetooth/bluetooth_dispatcher.cc b/content/renderer/bluetooth/bluetooth_dispatcher.cc
index d80430b70fdae72d054f076056b89d309e7e2479..d9391041bf7289146542cd6b1af24d6e0e885809 100644
--- a/content/renderer/bluetooth/bluetooth_dispatcher.cc
+++ b/content/renderer/bluetooth/bluetooth_dispatcher.cc
@@ -149,6 +149,7 @@ void BluetoothDispatcher::OnMessageReceived(const IPC::Message& msg) {
 }
 
 void BluetoothDispatcher::requestDevice(
+    int frame_routing_id,
     const WebRequestDeviceOptions& options,
     blink::WebBluetoothRequestDeviceCallbacks* callbacks) {
   int request_id = pending_requests_.Add(callbacks);
@@ -170,7 +171,8 @@ void BluetoothDispatcher::requestDevice(
   }
 
   Send(new BluetoothHostMsg_RequestDevice(CurrentWorkerId(), request_id,
-                                          filters, optional_services));
+                                          frame_routing_id, filters,
+                                          optional_services));
 }
 
 void BluetoothDispatcher::connectGATT(