LibWebView: Create a spare WebContent process

Since cross-site navigation is a pretty frequent task, creating a spare
process is commonplace in other browsers to reduce the overhead of
directing the target site to a new process.

We store this process on the WebView application. If it is unavailable,
we queue a task to create it later.

Libraries/LibWebView/Forward.h

@@ -10,6 +10,7 @@
 
 namespace WebView {
 
+class Application;
 class CookieJar;
 class Database;
 class InspectorClient;