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.

(Note that <rb> and <rtc> are new additions to HTML5 which currently
appear in the W3C but not the WHATWG version of the HTML5 spec.
Support for these has already been merged in gecko and webkit and
the editor plans to update the WHATWG spec.)

Bug: 67042
Change-Id: I8c0e65d782b6d23057a9723b87323b28e8bf8852
Merge "Allow HTML5 <rtc> tag (ruby support for East Asian typography)."