PostTask MaybeReleaseAdapter in BluetoothEventRouter::AdapterDiscoveringChanged

Calling MaybeReleaseAdapter directly from
BluetoothEventRouter::AdapterDiscoveringChanged could be a problem since
this can happen:
BluetoothAdapterBlueZ::AdapterPropertyChanged ->
BluetoothAdapterBlueZ::DiscoveringChanged ->
BluetoothEventRouter::AdapterDiscoveringChanged -> MaybeReleaseAdapter

If BluetoothEventRouter is the only pointer holder left then the caller
of BluetoothEventRouter::AdapterDiscoveringChanged could be gone even
before that function returns.

BUG=710216

Review-Url: https://codereview.chromium.org/2852773002
Cr-Commit-Position: refs/heads/master@{#468180}

extensions/browser/api/bluetooth/bluetooth_event_router.cc

@@ -329,8 +329,11 @@ void BluetoothEventRouter::AdapterDiscoveringChanged(
   DispatchAdapterStateEvent();
 
   // Release the adapter after dispatching the event.
-  if (!discovering)
-    MaybeReleaseAdapter();
+  if (!discovering) {
+    base::ThreadTaskRunnerHandle::Get()->PostTask(
+        FROM_HERE, base::Bind(&BluetoothEventRouter::MaybeReleaseAdapter,
+                              weak_ptr_factory_.GetWeakPtr()));
+  }
 }
 
 void BluetoothEventRouter::DeviceAdded(device::BluetoothAdapter* adapter,