Add missed file for the CMake build.

Bug: dawn:333
Change-Id: Ie966ec61f05c788bdb89a732bafe7f12c16e52cd
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15860
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>

src/Dummy.cpp

@@ -0,0 +1,18 @@
+// Copyright 2020 The Dawn Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// CMake requires that targets contain at least on file. This file is used when we want to create
+// empty targets.
+
+int someSymbolToMakeXCodeHappy = 0;