Cleanup: Fix base header include guards.

The header include guards should match the file path from root.

These entries were found using a modified version of Eric's
fix-include-guards.py script found attached in
https://code.google.com/p/chromium/issues/detail?id=435361#c7.

BUG=435361
TEST=base_unittests
[email protected]

Review URL: https://codereview.chromium.org/1137323003

Cr-Commit-Position: refs/heads/master@{#329731}
diff --git a/base/base64.h b/base/base64.h
index def9b67..dd72c39 100644
--- a/base/base64.h
+++ b/base/base64.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_BASE64_H__
-#define BASE_BASE64_H__
+#ifndef BASE_BASE64_H_
+#define BASE_BASE64_H_
 
 #include <string>
 
@@ -22,4 +22,4 @@
 
 }  // namespace base
 
-#endif  // BASE_BASE64_H__
+#endif  // BASE_BASE64_H_