Sort out <link rel="alternate stylesheet" title="">.sheet

[zcorpan]

See
web-platform-tests/wpt#2117
https://bugs.chromium.org/p/chromium/issues/detail?id=692560

From what I can tell, the check for empty (or absent) title was added in 5d5870f in response to @bzbarsky's comment in https://www.w3.org/Bugs/Public/show_bug.cgi?id=13977#c7

... I'm not wedded to this, offhand, but it's not quite clear what a browser should do with an alternate stylesheet with no title... (e.g. when is it enabled, if ever?)

Hixie:

Ok, I went with what Gecko does.

This check then seems to have unintentionally regressed in cc5fa75

See https://bugs.chromium.org/p/chromium/issues/detail?id=692560#c7 for the currently implemented landscape.

Given the lack of interop for this case, I suppose we can assume there aren't hard web compat constraints. So what do we want to do? Checking the attribute is cheap compared to downloading a stylesheet that then cannot be enabled. But maybe there are use cases for that?

cc @tkent-google @rune-opera

Setting the title to the selected/preferred set later will trigger the stylesheet to load in Gecko while it's readily available in Blink. I don't think that's a valid use case, though, as using a title in the first place and changing the selected set would be the way to accomplish that. I'd say we should go with the Gecko way.