[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/devserver_constants.py b/devserver_constants.py
index a1e4e7f..f2cdbb6 100644
--- a/devserver_constants.py
+++ b/devserver_constants.py
@@ -14,8 +14,11 @@
 GS_RELEASES_DIR = 'gs://chromeos-releases'
 GS_CHANNEL_DIR = GS_RELEASES_DIR + '/%(channel)s-channel/%(board)s/'
 
-VERSION_RE = 'R[-0-9\.]+'
 VERSION = '[-0-9\.]+'
+VERSION_RE = 'R%s' % VERSION
+
+STAGED_BUILD_REGEX = '/static/(?P<build>.*-.*/%s)/.*' % VERSION_RE
+
 
 #### Local storage locations and names. ####
 AUTOTEST_DIR = 'autotest'