IDNA: possibly use "domain to Unicode" in "valid domain"?

[domenic]

In #309 @rmisev noted that the extra flags were not set in the "valid domain" algorithm's call to ToUnicode. I pushed a fixup that adds them. But maybe it would be better to deduplicate the calls to ToUnicode, similar to what is already done for the calls to ToASCII.

This is not obviously straightforward since I am not sure who uses "domain to Unicode", and I'm not sure why it doesn't return failure on errors.

[annevk]

We don't have any users of "domain to Unicode" in the platform currently (after I removed origin to Unicode). #288 proposes to change that, but some at Chrome voiced concerns.

[annevk]

I ended up doing this in 0895123. The one call we have now is from the i18n address bar considerations, which seems okay.