[Android] Fix stack tool with provided symbols zip.

DEFAULT_SYMROOT was missed in https://codereview.chromium.org/18326020/
NOTRY=true

Review URL: https://chromiumcodereview.appspot.com/18920007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@211647 0039d316-1c4b-4281-b951-d872f2087c98

third_party/android_platform/development/scripts/stack

@@ -26,6 +26,8 @@ import subprocess
 import symbol
 import sys
 
+DEFAULT_SYMROOT='/tmp/symbols'
+
 def PrintUsage():
   """Print usage and exit with error."""
   # pylint: disable-msg=C6310