pepper_hash_for_uma: Allow multiple interfaces.

This change allows multiple interfaces to be specified on the command line,
improves documentation, and provides output similar to what's required in
tools/metrics/histograms/histograms.xml.

BUG=

Review URL: https://codereview.chromium.org/159283002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@250493 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 250493