Delete a DCHECK which triggers frequently in dev.

It's hit when creating a new window in fullscreen. The linked bug
mentions leaving it in place as a reminder to add a test, but I keep
getting bit. The bug should sufficiently document the need for a test.

Bug: 844482
Change-Id: Ibbd1cf24864c8122e82ba189072dcc2711afc326
Reviewed-on: https://chromium-review.googlesource.com/c/1482558
Auto-Submit: Sidney San Martín <sdy@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634705}

ui/views_bridge_mac/bridged_native_widget_impl.mm

@@ -815,10 +815,6 @@ void BridgedNativeWidgetImpl::OnWindowWillClose() {
 
 void BridgedNativeWidgetImpl::OnFullscreenTransitionStart(
     bool target_fullscreen_state) {
-  // Note: This can fail for fullscreen changes started externally, but a user
-  // shouldn't be able to do that if the window is invisible to begin with.
-  DCHECK(window_visible_);
-
   DCHECK_NE(target_fullscreen_state, target_fullscreen_state_);
   target_fullscreen_state_ = target_fullscreen_state;
   in_fullscreen_transition_ = true;