LibWeb: Handle scrolling an OutOfProcessWebView

When a mousewheel scroll event isn't handled by the web content
itself (e.g. an overflowed box or similar), the event needs to get
passed back up to the OutOfProcessWebView.

Userland/Services/WebContent/WebContentClient.ipc

@@ -8,6 +8,7 @@ endpoint WebContentClient = 90
     DidRequestCursorChange(i32 cursor_type) =|
     DidLayout(Gfx::IntSize content_size) =|
     DidChangeTitle(String title) =|
+    DidRequestScroll(int wheel_delta) =|
     DidRequestScrollIntoView(Gfx::IntRect rect) =|
     DidHoverLink(URL url) =|
     DidUnhoverLink() =|