WebContent: Allow the WebContent process to trigger repaints

After layout, we may want to repaint the page, so we now listen for the
PageClient::page_did_invalidate() notification and use it to drive a
client-side repaint.

Note that an invalidation request from LibWeb makes a full roundtrip
to the WebContent client and back since the client drives painting.

Services/WebContent/WebContentClient.ipc

@@ -1,6 +1,6 @@
 endpoint WebContentClient = 90
 {
     DidFinishLoad(URL url) =|
-
     DidPaint(Gfx::IntRect content_rect, i32 shbuf_id) =|
+    DidInvalidateContentRect(Gfx::IntRect content_rect) =|
 }