Fix an MQ parsing related crash

When MediaQueryParser used valuesList.clear(), clear() cleared the list's values, but not the offset of the current read value.
This fixes that.

BUG=353982

Review URL: https://codereview.chromium.org/204333005

git-svn-id: svn://svn.chromium.org/blink/trunk@169630 bbb929c8-8fbe-4397-9dbb-9b2b20218538