Fix proguard logic, set proguard config only when proguard is enabled.

build/android/ant/chromium-apk.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 default="error">
   <property name="ant.project.name" value="${APK_NAME}"/>
   <!--
     Gyp will pass CONFIGURATION_NAME as the target for ant to build. These targets will call the
     appropriate sdk tools target.
@@ skipping 22 matching lines @@
 
   <property-value name="target.abi" value="${APP_ABI}"/>
   <property name="resource.absolute.dir" value="${RESOURCE_DIR}"/>
   <property-value name="gen.absolute.dir" value="${out.dir}/gen"/>
   <property-location name="native.libs.absolute.dir" location="${out.dir}/libs"
       check-exists="false"/>
 
   <property-value name="version.code" value="${APP_MANIFEST_VERSION_CODE}"/>
   <property-value name="version.name" value="${APP_MANIFEST_VERSION_NAME}"/>
 
-  <property name="proguard.config" value="${PROGUARD_FLAGS}"/>
-  <property name="proguard.enabled" value="${PROGUARD_ENABLED}"/>
+  <!--
+    We use the PROGUARD_ENABLED flag for enabling proguard. By default proguard is enabled for
+    Release builds if proguard.config is set. Setting proguard.config even to an empty string will
+    enable proguard. Set this property only when we have explicitly enabled proguard.
+  -->
+  <condition property="proguard.config" value="${PROGUARD_FLAGS}">
+    <istrue value="${PROGUARD_ENABLED}"/>
+  </condition>
   <!-- TODO(shashishekhar): Enable emma and code-coverage filters. -->
 
   <condition property="asset.absolute.dir"
       value="${out.dir}/assets"
       else="${ASSET_DIR}">
     <equals arg1="${ASSET_DIR}" arg2=""/>
   </condition>
 
   <!-- Set the output directory for the final apk to the ${apks.dir}. -->
   <property-location name="out.final.file"
       location="${apks.dir}/${ant.project.name}-debug.apk"
       check-exists="false"/>
 
   <!-- Classpath for javac -->
   <path id="javac.custom.classpath">
     <filelist files="${INPUT_JARS_PATHS}"/>
   </path>
 
   <path id="out.dex.jar.input.ref">
     <path refid="javac.custom.classpath"/>
   </path>
 
   <import file="sdk-targets.xml"/>
   <import file="${sdk.dir}/tools/ant/build.xml"/>
 </project>