Added enable-cast to about:flags

The flag was already defined in chrome_switches. Added it to the list of
about:flags and the corresponding name and description.

BUG=345794,345326
NOTRY=true

Review URL: https://codereview.chromium.org/168693006

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252897 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252897