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/core/SkTraceEventCommon.h b/src/core/SkTraceEventCommon.h
index 57f6777..d066d90 100644
--- a/src/core/SkTraceEventCommon.h
+++ b/src/core/SkTraceEventCommon.h
@@ -1,6 +1,8 @@
 // Copyright 2015 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 SkTraceEventCommon_DEFINED
+#define SkTraceEventCommon_DEFINED
 
 // This header file defines the set of trace_event macros without specifying
 // how the events actually get collected and stored. If you need to expose trace
@@ -1037,3 +1039,4 @@
 #define TRACE_EVENT_SCOPE_NAME_GLOBAL ('g')
 #define TRACE_EVENT_SCOPE_NAME_PROCESS ('p')
 #define TRACE_EVENT_SCOPE_NAME_THREAD ('t')
+#endif  // SkTraceEventCommon_DEFINED