Bug fix - rem & ch in MQs/sizes treated as unknown units

'rem' and 'ch' units were unrecognized when used inside MQs/sizes, since they were not in the unit table.
I've added them and fixed another bug related to CH and MQs.

I've also added the 'fr' and 'turn' units to the unit table, even though there aren't valid MQs that can use them,
since some other code may use the unit table in the future, and they should be there for completeness.

BUG=398185

Review URL: https://codereview.chromium.org/427683002

git-svn-id: svn://svn.chromium.org/blink/trunk@179103 bbb929c8-8fbe-4397-9dbb-9b2b20218538