Merge bools into bit fields in TextTrackCue and VTTCue

On x86_64 Linux, this reduces sizeof(VTTCue) from 248 to 232.
This does likely not result in real runtime savings, but is a
step back from 256, where a real memory increase is likely.

BUG=none

Review URL: https://codereview.chromium.org/104243003

git-svn-id: svn://svn.chromium.org/blink/trunk@163183 bbb929c8-8fbe-4397-9dbb-9b2b20218538