[devserver] Update staged.timestamp when accessing static content.

When accessing static content, the staged.timestamp file will now
be updated to reflect the fact the data was recently accessed.

This only impacts content accessed through Cherrypy not through
Apache.

BUG=chromium:390228
TEST=Verified stage.timestamp is updated when content is accessed
directly. Ran a suite on Moblab using this change.

Change-Id: Ic8c69c10d0f2d5fbce3b5c1372fb27ca18b025d8
Reviewed-on: https://chromium-review.googlesource.com/215132
Reviewed-by: Simran Basi <sbasi@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
diff --git a/downloader.py b/downloader.py
index c02ebe2..9174f10 100755
--- a/downloader.py
+++ b/downloader.py
@@ -95,7 +95,7 @@
     return os.path.join(static_dir, rel_path, build)
 
   @staticmethod
-  def _TouchTimestampForStaged(directory_path):
+  def TouchTimestampForStaged(directory_path):
     file_name = os.path.join(directory_path, Downloader._TIMESTAMP_FILENAME)
     # Easiest python version of |touch file_name|
     with file(file_name, 'a'):
@@ -176,7 +176,7 @@
 
     # We are doing some work on this build -- let's touch it to indicate that
     # we shouldn't be cleaning it up anytime soon.
-    Downloader._TouchTimestampForStaged(self._build_dir)
+    Downloader.TouchTimestampForStaged(self._build_dir)
 
     # Create factory to create build_artifacts from artifact names.
     build = self.ParseUrl(self._archive_url)[1]