bluetooth: Remove DCHECK in destructor

The DCHECK in ~Bluetooth() is reachable because navigator.bluetooth can
be destroyed before the execution context is destroyed.

Bug: 972058
Change-Id: I5ed4d4c3b874a6bb7b73e91efcf918945dbf4250
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2591240
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Commit-Queue: Kentaro Hara <haraken@chromium.org>
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#836885}

third_party/blink/renderer/modules/bluetooth/bluetooth.cc

@@ -467,9 +467,7 @@ Bluetooth::Bluetooth(Navigator& navigator)
       client_receivers_(this, navigator.DomWindow()),
       service_(navigator.DomWindow()) {}
 
-Bluetooth::~Bluetooth() {
-  DCHECK(client_receivers_.empty());
-}
+Bluetooth::~Bluetooth() = default;
 
 BluetoothDevice* Bluetooth::GetBluetoothDeviceRepresentingDevice(
     mojom::blink::WebBluetoothDevicePtr device_ptr,