DeleteBranches: Split static Input class out to extension API

Change-Id: I359b6a7827d81c3a87900a5d6680d1df5d4e9ba6

gerrit-extension-api/src/main/java/com/google/gerrit/extensions/api/projects/DeleteBranchesInput.java

@@ -0,0 +1,21 @@
+// Copyright (C) 2016 The Android Open Source Project
+//
+// 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.gerrit.extensions.api.projects;
+
+import java.util.List;
+
+public class DeleteBranchesInput {
+  public List<String> branches;
+}