Fix Location.SetHref for Iframe with no src

[gterzian]

test: "/html/browsers/browsing-the-web/navigating-across-documents/005.html"
code:

servo/components/script/dom/location.rs

Line 165 in d23bc4f

Err(e) => return Err(Error::Type(format!("Couldn't parse URL: {}", e))),

spec: https://html.spec.whatwg.org/multipage/#attr-iframe-src

Current error: Error at http://web-platform.test:8000/html/browsers/browsing-the-web/navigating-across-documents/005.html:1:1 Couldn't parse URL: relative URL with a cannot-be-a-base base

I guess one would have to parse the url related to the parent window when there is no src?
Or perhaps it's the test that should be updated, because the spec seems to read that the src should default to "about:blank" if unset.. https://html.spec.whatwg.org/multipage/#otherwise-steps-for-iframe-or-frame-elements

[jdm]

That's a fascinating test! For clarity:

• there is an iframe that is pointed at about:blank
• there is an <a> element with an onclick that sets the iframe's contentWindow.location to a relative URL
• this makes Location::SetHref unhappy because about:blank is not regarded as a valid base URL for relative URLs

[jdm]

The problem here may actually be the implementation of Location::SetHref. The spec says to use the entry settings object as the base URL, while we're using the location's window object's URL. We should be using the entry_global API instead.

[jdm]

#21926 was an attempt to fix this that hit some surprising test failures that need to be investigated.