Make AppWindowWaiter a common test support

apps/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 import("//build/config/features.gni")
 import("//build/config/ui.gni")
 import("//extensions/features/features.gni")
 
 assert(!is_android && !is_ios)
 
@@ skipping 53 matching lines @@
       "//ui/views",
     ]
     if (enable_extensions) {
       deps += [ "//extensions/browser" ]
     }
   }
 
   # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
   configs += [ "//build/config/compiler:no_size_t_to_int_warning" ]
 }
+
+static_library("test_support") {
+  testonly = true
+  sources = [
+    "test/app_window_waiter.cc",
+    "test/app_window_waiter.h",
+  ]
+
+  public_deps = [
+    "//content/public/browser",
+  ]
+}