Improve Spec compliance of the iframe "name" attribute

[gterzian]

Current behavior:

Currently, Iframe.SetName only "sets the name" if the iframe has a browsing context/window proxy:

servo/components/script/dom/htmliframeelement.rs

Line 473 in bf66767

if let Some(window) = self.GetContentWindow() {

So when the "name" attribute is mutated before the iframe is inserted in the document, this has no effect:

servo/components/script/dom/htmliframeelement.rs

Line 527 in bf66767

&local_name!("name") => {

Tests:

Some tests assume the attribute can be mutated "with effect" before the iframe has been inserted in the document:

servo/tests/wpt/web-platform-tests/html/browsers/windows/noreferrer-window-name.html

Line 43 in bf66767

ifr.name = "sufficientlyrandomwindownameamiright2";

There might be other tests testing this more specifically, or such a test could be written otherwise.

Desired behavior:

Per the spec "When the browsing context is created, if the attribute is present, the browsing context name must be set to the value of this attribute;", and after that "Whenever the name attribute is set, the nested browsing context's name must be changed to the new value. If the attribute is removed, the browsing context name must be set to the empty string." https://html.spec.whatwg.org/multipage/iframe-embed-object.html#attr-iframe-name

"When an iframe element is inserted into a document that has a browsing context, the user agent must create a new browsing context, set the element's nested browsing context to the newly-created browsing context, and then process the iframe attributes for the "first time". https://html.spec.whatwg.org/multipage/iframe-embed-object.html#the-iframe-element

Currently, "process_the_iframe_attributes" doesn't involve any previously set name, so it should be updated to set the name of the newly created browsing context to be the "name" attribute of the iframe:

servo/components/script/dom/htmliframeelement.rs

Line 206 in bf66767

fn process_the_iframe_attributes(&self, mode: ProcessingMode) {