LibWeb: Allow construction of PaintingSurface directly from IOSurface

Previously, constructing a PaintingSurface from an IOSurface required
wrapping IOSurface into a Metal texture before passing it to the
PaintingSurface constructor. This process was cumbersome, as the caller
needed access to a MetalContext to perform the wrapping.

With this change SkiaBackendContext maintains a reference to the
MetalContext which makes it possible to do:
IOSurface -> MetalTexture -> SkSurface within a PaintingSurface
constructor.

Libraries/LibWeb/HTML/TraversableNavigable.h

@@ -149,7 +149,7 @@ private:
     RefPtr<Gfx::SkiaBackendContext> m_skia_backend_context;
 
 #ifdef AK_OS_MACOS
-    OwnPtr<Gfx::MetalContext> m_metal_context;
+    RefPtr<Gfx::MetalContext> m_metal_context;
 #endif
 };