A little clean up of encoding labels

Drop CP864 (PC Arabic) and JIS7 (as an alias to ISO-2022-JP)
because neither is listed in the WHATWG Encoding standard.


Drop a couple of aliases macintosh (macroman). They're already listed in
the ICU alias table.

BUG=NONE
TEST=NONE

Review URL: https://codereview.chromium.org/222893003

git-svn-id: svn://svn.chromium.org/blink/trunk@170809 bbb929c8-8fbe-4397-9dbb-9b2b20218538