Split out common parsing code for Position/Size VTT cue settings

While in the vicinity, also reposition the braces for the cases in
the switch.

BUG=305317

Review URL: https://codereview.chromium.org/118263002

git-svn-id: svn://svn.chromium.org/blink/trunk@164095 bbb929c8-8fbe-4397-9dbb-9b2b20218538