Added Google Cache Invalidation library to DEPS and got it to compile

third_party/cacheinvalidation/overrides/google/cacheinvalidation/string_util.h

Index: third_party/cacheinvalidation/overrides/google/cacheinvalidation/string_util.h
diff --git a/third_party/cacheinvalidation/overrides/google/cacheinvalidation/string_util.h b/third_party/cacheinvalidation/overrides/google/cacheinvalidation/string_util.h
new file mode 100644
index 0000000000000000000000000000000000000000..81e9d8ba5b1d99aa50c83b754d07f8753b14029e
--- /dev/null
+++ b/third_party/cacheinvalidation/overrides/google/cacheinvalidation/string_util.h
@@ -0,0 +1,16 @@
+// Copyright (c) 2010 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.
+
+#ifndef GOOGLE_CACHEINVALIDATION_STRING_UTIL_H_
+#define GOOGLE_CACHEINVALIDATION_STRING_UTIL_H_
+
+#include "base/string_util.h"
+
+namespace invalidation {
+
+using ::StringAppendV;
+
+}  // namespace invalidation
+
+#endif  // GOOGLE_CACHEINVALIDATION_STRING_UTIL_H_