LibWeb+WebContent: Change event loop to synchronously paint next frame

...instead of scheduling repaint timer in PageClient.

This change fixes flickering on Discord that happened because:
- Event loop schedules repainting by activating repaint timer
- `Document::tear_down_layout_tree()` destroys paintable tree
- Repaint timer invokes callback and renders an empty frame because
  paintable tree was destroyed

Userland/Libraries/LibWeb/SVG/SVGDecodedImageData.h

@@ -73,6 +73,7 @@ public:
     virtual double device_pixels_per_css_pixel() const override { return 1.0; }
     virtual CSS::PreferredColorScheme preferred_color_scheme() const override { return m_host_page->client().preferred_color_scheme(); }
     virtual void request_file(FileRequest) override { }
+    virtual void paint_next_frame() override { }
     virtual void paint(DevicePixelRect const&, Gfx::Bitmap&, Web::PaintOptions = {}) override { }
     virtual void schedule_repaint() override { }
     virtual bool is_ready_to_paint() const override { return true; }