Split off histograms.xml from the main metrics WATCHLISTS entry.

I'm finding that I get lots of watchlist spam from histograms.xml changes that
in practice I just ignore.  Therefore, I've split the watchlists entry into two:
one for non-XML files, and one for XML files.

BUG=none
TEST=none
R=asvitkine@chromium.org

Review URL: https://codereview.chromium.org/168263007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252482 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252482