Use TextTrackMode and TextTrackKind enums in TextTrack IDL

The generated code (modeAttributeSetter in V8TextTrack.cpp) now
checks that the string is one of the three allowed values, so we
can just assert it internally.

Also fiddle a bit with the IDL to make it match the spec order.

BUG=313602

Review URL: https://codereview.chromium.org/52593009

git-svn-id: svn://svn.chromium.org/blink/trunk@161055 bbb929c8-8fbe-4397-9dbb-9b2b20218538