Treat failure to fetch a master as fatal.

This patch modifies fetch_buildbot_data to treat a failure to
fetch from a master as fatal. The caller can override this
behavior with a boolean to force an update.

For admins' convenience, now there's also a "force" parameter
for /updatebuilders which controls this flag. Otherwise
/updatebuilders defaults to aborting the entire fetch when
the server fails to fetch a master's data.

Update unit tests to exercise this behavior as well. Also the
tests now use sets to compare lists of fetched URLs to avoid
relying on the specific ordering of expected results.

BUG=305364

Review URL: https://codereview.chromium.org/26135003

git-svn-id: svn://svn.chromium.org/blink/trunk@159739 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Use DEFINE_NODE_TYPE_CASTS instead of using manual toHTMLFooElement()

Replace manual toHTMLFooElement() with DEFINE_NODE_TYPE_CASTS(). It supports more
various helper functions as well as we can use it easily. However, this cl only can
cover toHTMLFooElement() except for non-support toHTMLFooElement().

BUG=N/A

Review URL: https://codereview.chromium.org/27142002

git-svn-id: svn://svn.chromium.org/blink/trunk@159740 bbb929c8-8fbe-4397-9dbb-9b2b20218538