Use DOMString for internal data of Trusted Types (#37472)

This avoids various conversions back and forth between DOMString and
String. By using DOMString consistently, we avoid these
double-conversions. The only caveat are the USVString which are
initially passed into
TrustedScriptURL.

Part of #36258

Signed-off-by: Tim van der Lippe <tvanderlippe@gmail.com>

components/script/dom/htmliframeelement.rs

@@ -622,7 +622,11 @@ impl HTMLIFrameElementMethods<crate::DomTypeHolder> for HTMLIFrameElement {
             can_gc,
         )?;
         // Step 2: Set an attribute value given this, srcdoc's local name, and compliantString.
-        element.set_attribute(local_name, AttrValue::String(value), can_gc);
+        element.set_attribute(
+            local_name,
+            AttrValue::String(value.as_ref().to_owned()),
+            can_gc,
+        );
         Ok(())
     }