Fix Camera3D crash with preload

When preloading, the initial Camera3D that was created does
not have an associated impl() (i.e. an input::Camera3D)
because an input manager is not instantiated in preload.

This change allows graceful transition from the placeholder
dom::Camera3D without input to a dom::Camera3D with input.

b/198526160

Change-Id: Ib65355a4205c971f37c5499b599dc877975acadc
(cherry picked from commit 9cb5d4454339405ff6beff41040479f087db0051)

cobalt/dom/window.cc

@@ -630,9 +630,11 @@ void Window::SetSize(ViewportSize size) {
 }
 
 void Window::UpdateCamera3D(const scoped_refptr<input::Camera3D>& camera_3d) {
-  if (camera_3d_) {
+  if (camera_3d_ && camera_3d_->impl()) {
+    // Update input object for existing camera.
     camera_3d_->impl()->SetInput(camera_3d);
   } else {
+    // Create a new camera which uses the given input camera object.
     camera_3d_ = new Camera3D(camera_3d);
     camera_3d_->StartOrientationEvents(base::AsWeakPtr(this));
   }