Rename TimeRemainingShort() --> TimeDurationShort()

For consistency with TimeDurationLong().

The term TimeDuration denotes an interval, eg. "1 hour".  On the
other hand, TimeRemaining indicates an interval until something
happens, eg. "1 hour left".

BUG=
TBR=asanka@chromium.org (download_item_model.cc), jhawkins@chromium.org (policy_ui.cc)

Review URL: https://codereview.chromium.org/144643006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@248969 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 248969