Implement WebBluetooth getDescriptor[s]

third_party/WebKit/LayoutTests/bluetooth/getDescriptor/descriptor-not_found.html

Index: third_party/WebKit/LayoutTests/bluetooth/getDescriptor/descriptor-not_found.html
diff --git a/third_party/WebKit/LayoutTests/bluetooth/notifications/stop-without-starting.html b/third_party/WebKit/LayoutTests/bluetooth/getDescriptor/descriptor-not_found.html
similarity index 55%
copy from third_party/WebKit/LayoutTests/bluetooth/notifications/stop-without-starting.html
copy to third_party/WebKit/LayoutTests/bluetooth/getDescriptor/descriptor-not_found.html
index 7cbb02923277a8249db0a1bc80b9830129cd2025..05e9db9491b2854b3f8fc12b4818ee0450dd8c69 100644
--- a/third_party/WebKit/LayoutTests/bluetooth/notifications/stop-without-starting.html
+++ b/third_party/WebKit/LayoutTests/bluetooth/getDescriptor/descriptor-not_found.html
@@ -4,13 +4,19 @@
 <script src="../../resources/bluetooth/bluetooth-helpers.js"></script>
 <script>
 'use strict';
+
 promise_test(() => {
   return setBluetoothFakeAdapter('HeartRateAdapter')
     .then(() => requestDeviceWithKeyDown({
-      filters: [{services: ['heart_rate']}]}))
+      filters: [{services: ['heart_rate']}],
+      optionalServices: ['generic_access']}))
     .then(device => device.gatt.connect())
     .then(gattServer => gattServer.getPrimaryService('heart_rate'))
     .then(service => service.getCharacteristic('heart_rate_measurement'))
-    .then(characteristic => characteristic.stopNotifications());
-}, "Stop without starting.");
+    .then(characteristic => assert_promise_rejects_with_message(
+        characteristic.getDescriptor(0x0101) /* some random value that doesn't exist*/,
+      new DOMException(
+          'No Descriptors with specified UUID found in Characteristic.',
+          'NotFoundError')));
+}, 'Request for absent descriptor. Reject with NotFoundError.');
 </script>