LibWeb: Add missing includes

This remained undetected for a long time as HeaderCheck is disabled by
default. This commit makes the following file compile again:

    // file: compile_me.cpp
    #include <LibWeb/HTML/CrossOrigin/CrossOriginOpenerPolicy.h>
    // That's it, this was enough to cause a compilation error.

Likewise for most other files touched by this commit.

Userland/Libraries/LibWeb/HTML/CrossOrigin/CrossOriginOpenerPolicy.h

@@ -6,6 +6,8 @@
 
 #pragma once
 
+#include <AK/String.h>
+
 namespace Web::HTML {
 
 // https://html.spec.whatwg.org/multipage/origin.html#cross-origin-opener-policy-value

Userland/Libraries/LibWeb/HTML/CrossOrigin/CrossOriginOpenerPolicyEnforcementResult.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/URL.h>
+#include <LibWeb/HTML/CrossOrigin/CrossOriginOpenerPolicy.h>
 #include <LibWeb/HTML/Origin.h>
 
 namespace Web::HTML {