base: Fix/add header #ifndef guards.

base/allocator/allocator_extension.h

Index: base/allocator/allocator_extension.h
diff --git a/base/allocator/allocator_extension.h b/base/allocator/allocator_extension.h
index de3119f85e66609f9ad0f7c97a29f6d326993331..e65822b359e010a28150333a6bf19c2dd8be47a1 100644
--- a/base/allocator/allocator_extension.h
+++ b/base/allocator/allocator_extension.h
@@ -2,8 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#ifndef BASE_ALLOCATOR_ALLOCATOR_EXTENSION_H
-#define BASE_ALLOCATOR_ALLOCATOR_EXTENSION_H
+#ifndef BASE_ALLOCATOR_ALLOCATOR_EXTENSION_H_
+#define BASE_ALLOCATOR_ALLOCATOR_EXTENSION_H_
 
 #include <stddef.h> // for size_t
 
@@ -56,4 +56,4 @@ BASE_EXPORT void SetReleaseFreeMemoryFunction(
 }  // namespace allocator
 }  // namespace base
 
-#endif
+#endif  // BASE_ALLOCATOR_ALLOCATOR_EXTENSION_H_