Refuse to always clobber.

build/android/buildbot.sh

 #!/bin/bash
 # Copyright (c) 2011 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.
 #
 # "compile and run tests" script for the android build of chromium.
 # Intended for use by buildbot.
 # At this time, we only have one bot which is both a builder and
 # tester.  Script assumes it runs in the "build" directory.
 #
@@ skipping 13 matching lines @@
 export ANDROID_SDK_ROOT=/usr/local/google/android-sdk-linux_x86
 export ANDROID_NDK_ROOT=/usr/local/google/android-ndk-r7
 for mandatory_directory in "${ANDROID_SDK_ROOT}" "${ANDROID_NDK_ROOT}" ; do
   if [[ ! -d "${mandatory_directory}" ]]; then
     echo "Directory ${mandatory_directory} does not exist."
     echo "Build cannot continue."
     exit 1
   fi
 done
 
-if [ ! -n "$BUILDBOT_CLOBBER" ]; then
+if [ ! "$BUILDBOT_CLOBBER" = "" ]; then
   NEED_CLOBBER=1
 fi
 
 ## Build and test steps
 
 echo "@@@BUILD_STEP Configure with envsetup.sh@@@"
 . build/android/envsetup.sh
 
 if [ "$NEED_CLOBBER" -eq 1 ]; then
   echo "@@@BUILD_STEP Clobber@@@"
   rm -rf "${SRC_ROOT}"/out
 fi
 
 echo "@@@BUILD_STEP android_gyp@@@"
 android_gyp
 
 echo "@@@BUILD_STEP Compile@@@"
 make -j4
 
 echo "@@@BUILD_STEP Run Tests@@@"
 build/android/run_tests.py -e --xvfb
 
 exit 0