Allow HTML5 <rtc> tag (ruby support for East Asian typography).

We currently allow <ruby>, <rt>, <rb>, and <rp> but not the W3C HTML5 <rtc>
element.  Fix that.

Bug: 67042
Change-Id: I0fc5a759528be2491f31c2fe6adb129c0afd3cc8
Merge "Allow HTML5 <rtc> tag (ruby support for East Asian typography)."