Convert more DOM_NO_ERR cases to assertions

There is some risk to this change. The first is that I made a mistake. The
other is that one of the APIs that doesn't currently return an error changes
in the future.

src/cdp/domains/page.zig

@@ -17,7 +17,6 @@
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
 const std = @import("std");
-const URL = @import("../../url.zig").URL;
 const Page = @import("../../browser/page.zig").Page;
 const Notification = @import("../../notification.zig").Notification;