Initialize VTTCue::m_displaySize to undefinedSize

It was not initialized at all, and while the current code appears
to always write it before reading it, it's a bug waiting to happen.

(It would be nice to not have render state in VTTCue at all.)

BUG=none

Review URL: https://codereview.chromium.org/132563005

git-svn-id: svn://svn.chromium.org/blink/trunk@165987 bbb929c8-8fbe-4397-9dbb-9b2b20218538