Turn on build/header_guard in cpplint.

Set the correct root path in CPPLINT.cfg and fix existing violations.

Test: tools/repohooks/tools/cpplint.py --quiet system/update_engine/**/*.h
Change-Id: I37812854a68116c69bcef28f8b15856b89a07c4d
diff --git a/power_manager_chromeos.h b/power_manager_chromeos.h
index ad49889..eeb14d8 100644
--- a/power_manager_chromeos.h
+++ b/power_manager_chromeos.h
@@ -14,8 +14,8 @@
 // limitations under the License.
 //
 
-#ifndef UPDATE_ENGINE_POWER_MANAGER_H_
-#define UPDATE_ENGINE_POWER_MANAGER_H_
+#ifndef UPDATE_ENGINE_POWER_MANAGER_CHROMEOS_H_
+#define UPDATE_ENGINE_POWER_MANAGER_CHROMEOS_H_
 
 #include <base/macros.h>
 #include <power_manager/dbus-proxies.h>
@@ -41,4 +41,4 @@
 
 }  // namespace chromeos_update_engine
 
-#endif  // UPDATE_ENGINE_POWER_MANAGER_H_
+#endif  // UPDATE_ENGINE_POWER_MANAGER_CHROMEOS_H_