Refuse to always clobber.

BUG=
TEST=

Review URL: http://codereview.chromium.org/8624003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@111017 0039d316-1c4b-4281-b951-d872f2087c98

build/android/buildbot.sh

@@ -31,7 +31,7 @@ for mandatory_directory in "${ANDROID_SDK_ROOT}" "${ANDROID_NDK_ROOT}" ; do
   fi
 done
 
-if [ ! -n "$BUILDBOT_CLOBBER" ]; then
+if [ ! "$BUILDBOT_CLOBBER" = "" ]; then
   NEED_CLOBBER=1
 fi