Add missing license headers

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=263591284

Author: cushon

core/src/test/java/com/google/googlejavaformat/java/StringWrapperTest.java

@@ -1,3 +1,17 @@
+/*
+ * Copyright 2019 Google Inc.
+ *
+ * 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.
+ */
+
 package com.google.googlejavaformat.java;
 
 import static com.google.common.truth.Truth.assertThat;