html/atom: add atom.Rb and atom.Rtc

Updates golang/go#23071

Change-Id: I07aae04757e83a3a03681a2ce92e4cab194ef64a
Reviewed-on: https://go-review.googlesource.com/107198
Run-TryBot: Kunpei Sakai <namusyaka@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Nigel Tao <nigeltao@golang.org>

Author: namusyaka

html/atom/gen.go

@@ -306,7 +306,7 @@ func (t *table) push(i uint32, depth int) bool {
 
 // The lists of element names and attribute keys were taken from
 // https://html.spec.whatwg.org/multipage/indices.html#index
-// as of the "HTML Living Standard - Last Updated 18 September 2017" version.
+// as of the "HTML Living Standard - Last Updated 16 April 2018" version.
 
 // "command", "keygen" and "menuitem" have been removed from the spec,
 // but are kept here for backwards compatibility.
@@ -701,6 +701,8 @@ var extra = []string{
 	"plaintext",
 	"prompt",
 	"public",
+	"rb",
+	"rtc",
 	"spacer",
 	"strike",
 	"svg",