Make ant touch jar outputs

build/android/ant/chromium-jars.xml

 <!--
   Copyright (c) 2012 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.
 -->
 <project name="chromium-jars" default="dist">
   <!--
     Common ant build file for for chromium_*.jars.
     For creating a new chromium_*.jar :
     1. Use build/java.gypi action.
@@ skipping 47 matching lines @@
       debug="true"
       includeantruntime="false"
     />
   </target>
 
   <target name="dist" depends="compile"
       description="Generate chromium_${PACKAGE_NAME}.jar.">
     <!-- Create the distribution directory -->
     <mkdir dir="${lib.dir}" />
     <jar jarfile="${lib.dir}/chromium_${PACKAGE_NAME}.jar" basedir="${dest.dir}"/>
+
+    <!-- If Gyp thinks this output is stale but Ant doesn't, the modification
+         time should still be updated. Otherwise, this target will continue to
+         be rebuilt in future builds.
+    -->
+    <touch file="${lib.dir}/chromium_${PACKAGE_NAME}.jar"/>
   </target>
 
   <target name="clean" description="clean up">
     <!-- Delete the appropriate directory trees -->
     <delete dir="${dest.dir}" />
   </target>
 </project>