Auto merge of #11544 - jdm:privatebrowsing, r=asajeffrey

Implement private browsing for mozbrowser

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Support the `mozprivatebrowsing` attribute on mozbrowser iframes. This separates the non-private and private sessions in terms of cookies, HSTS lists, cached HTTP credentials, HTTP connection pools, and web storage. The private session is shared between all private mozbrowsers, and lasts until shutdown.

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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] There are tests for these changes

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components/script/dom/htmliframeelement.rs

@@ -562,6 +562,21 @@ impl HTMLIFrameElementMethods for HTMLIFrameElement {
     make_getter!(Height, "height");
     // https://html.spec.whatwg.org/multipage/#dom-dim-height
     make_dimension_setter!(SetHeight, "height");
+
+    // check-tidy: no specs after this line
+    fn SetMozprivatebrowsing(&self, value: bool) {
+        let element = self.upcast::<Element>();
+        element.set_bool_attribute(&Atom::from("mozprivatebrowsing"), value);
+    }
+
+    fn Mozprivatebrowsing(&self) -> bool {
+        if window_from_node(self).is_mozbrowser() {
+            let element = self.upcast::<Element>();
+            element.has_attribute(&Atom::from("mozprivatebrowsing"))
+        } else {
+            false
+        }
+    }
 }
 
 impl VirtualMethods for HTMLIFrameElement {