WebContent: Basic scrolling support! :^)

The WebContentView widget now inherits from GUI::ScrollableWidget and
will pass its scroll offset over to the WebContent process as it's
changing. This is not super efficient and can get a bit laggy, but it
will do fine as an initial scrolling implementation.

Just like the single-process Web::PageView widget, WebContentView also
paints scrolled content by translating the Gfx::Painter.

Services/WebContent/WebContentClient.ipc

@@ -4,4 +4,5 @@ endpoint WebContentClient = 90
     DidPaint(Gfx::IntRect content_rect, i32 shbuf_id) =|
     DidInvalidateContentRect(Gfx::IntRect content_rect) =|
     DidChangeSelection() =|
+    DidLayout(Gfx::IntSize content_size) =|
 }