[bluetooth] Fix BluetoothTest.ConstructDefaultAdapter on macOS

This test became flaky on macOS after r763057 because the code to
initialize the adapter when calling BluetoothTest::WithDefaultAdapter()
was accidentally removed on macOS. This patch restores this logic,
copying what is done on other platforms.

Bug: 1076005
Change-Id: If709274183b668022d55c69f8b9d2a09a9c7dd1a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2171442
Auto-Submit: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Ovidio de Jesús Ruiz-Henríquez <odejesush@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#763587}

device/bluetooth/bluetooth_adapter_unittest.cc

@@ -691,8 +691,7 @@ TEST_F(BluetoothAdapterTest, StartDiscoverySessionError_Destroy) {
 }
 
 // TODO(scheib): Enable BluetoothTest fixture tests on all platforms.
-// TODO(https://crbug.com/1076005) Flaky on Mac
-#if defined(OS_ANDROID)
+#if defined(OS_ANDROID) || defined(OS_MACOSX)
 #define MAYBE_ConstructDefaultAdapter ConstructDefaultAdapter
 #else
 #define MAYBE_ConstructDefaultAdapter DISABLED_ConstructDefaultAdapter

device/bluetooth/test/bluetooth_test_mac.mm

@@ -111,6 +111,10 @@ void BluetoothTestMac::InitWithDefaultAdapter() {
   auto adapter = BluetoothAdapterMac::CreateAdapter();
   adapter_mac_ = adapter.get();
   adapter_ = std::move(adapter);
+
+  base::RunLoop run_loop;
+  adapter_->Initialize(run_loop.QuitClosure());
+  run_loop.Run();
 }
 
 void BluetoothTestMac::InitWithoutDefaultAdapter() {