Turn off checkstyle for protobuf generated code.

- Use protobuf generated comment to turn off checkstyle,
  so that we don't have to keep updateing exclusion rules.

Change-Id: Ic10d5576b104d67c2b83114741ced0768d09fca2
diff --git a/conf/checkstyle/sun_checks.xml b/conf/checkstyle/sun_checks.xml
index 4993209..5858af8 100644
--- a/conf/checkstyle/sun_checks.xml
+++ b/conf/checkstyle/sun_checks.xml
@@ -92,7 +92,7 @@
     <!-- </module> -->
 
     <module name="SuppressionCommentFilter">
-        <property name="offCommentFormat" value="CHECKSTYLE:OFF"/>
+        <property name="offCommentFormat" value="(CHECKSTYLE\:OFF|Generated by the protocol buffer compiler.)"/>
         <property name="onCommentFormat" value="CHECKSTYLE:ON"/>
     </module>
     <module name="TreeWalker">