Add a script to delete nightly test data that are old.

TEST=tested on tc-build.hot

Change-Id: I6b6bc298b56c6ac79cbbd66f8168bc234580c884
Reviewed-on: https://chrome-internal-review.googlesource.com/150264
Reviewed-by: Yunlian Jiang <yunlian@google.com>
Commit-Queue: Han Shen <shenhan@google.com>
Tested-by: Han Shen <shenhan@google.com>
diff --git a/build_tc.py b/build_tc.py
index 02918b6..5419b55 100755
--- a/build_tc.py
+++ b/build_tc.py
@@ -38,7 +38,7 @@
         "etc/portage/package.mask/cross-%s" % self._ctarget)
     self._new_mask_file = None
 
-    self._chroot_source_path = os.path.join(constants.mounted_toolchain_root,
+    self._chroot_source_path = os.path.join(constants.MOUNTED_TOOLCHAIN_ROOT,
                                             self._name).lstrip("/")
     self._incremental = incremental
     self._build_env = build_env