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/src/codec/SkCodecImageGenerator.h b/src/codec/SkCodecImageGenerator.h
index e1aa220..51086eb 100644
--- a/src/codec/SkCodecImageGenerator.h
+++ b/src/codec/SkCodecImageGenerator.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 SkCodecImageGenerator_DEFINED
+#define SkCodecImageGenerator_DEFINED
 
 #include "SkCodec.h"
 #include "SkColorTable.h"
@@ -47,3 +49,4 @@
 
     typedef SkImageGenerator INHERITED;
 };
+#endif  // SkCodecImageGenerator_DEFINED