LibWeb+WebContent: Add a simple API for running arbitrary JavaScript

This patch adds OutOfProcessWebView::run_javascript(StringView).
This can be used by the OOPWV embedder to execute arbitrary JavaScript
in the top-level browsing context on the WebContent process side.

Userland/Services/WebContent/WebContentServer.ipc

@@ -30,6 +30,8 @@ endpoint WebContentServer
     js_console_initialize() =|
     js_console_input(String js_source) =|
 
+    run_javascript(String js_source) =|
+
     get_selected_text() => (String selection)
     select_all() =|
 }