Make header files idempotent; script to check

Change-Id: I960ded854e6bc7cdee029a7393cac2a686c41754
Reviewed-on: https://skia-review.googlesource.com/8308
Commit-Queue: Hal Canary <halcanary@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
diff --git a/tests/CodecPriv.h b/tests/CodecPriv.h
index 515dbde..50c205c 100644
--- a/tests/CodecPriv.h
+++ b/tests/CodecPriv.h
@@ -4,6 +4,8 @@
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
+#ifndef CodecPriv_DEFINED
+#define CodecPriv_DEFINED
 
 #include "SkBitmap.h"
 #include "SkCodec.h"
@@ -32,3 +34,4 @@
             bm->rowBytes(), nullptr, colorPtr, colorCountPtr);
     return result == SkCodec::kSuccess || result == SkCodec::kIncompleteInput;
 }
+#endif  // CodecPriv_DEFINED