Check variable PRODUCT_DIR firstly for content shell test

Simple fix like other build files

BUG=
TEST=


Review URL: https://chromiumcodereview.appspot.com/10825033

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148942 0039d316-1c4b-4281-b951-d872f2087c98

content/shell/android/javatests/content_shell_test_apk.xml

@@ -34,18 +34,13 @@
 
   <!-- We expect PRODUCT_DIR to be set like the gyp var
        (e.g. $ROOT/out/Debug) -->
-  <!-- TODO(satish): ideally we need this to run before -build-setup, where
-       directories are made based on this variable. -->
-  <target name="-pre-build">
-    <if>
-      <condition>
-        <isset property="PRODUCT_DIR" />
-      </condition>
-      <else>
-        <fail message="PRODUCT_DIR env var not set?" />
-      </else>
-    </if>
-  </target>
+  <fail message="PRODUCT_DIR env var not set?">
+    <condition>
+      <not>
+        <isset property="PRODUCT_DIR"/>
+      </not>
+    </condition>
+  </fail>
 
   <target name="-post-compile">
     <!-- copy gdbserver to main libs directory if building debug.