ash: Expect null window in OutputProtectionDelegate

OutputProtectionImpl may create (via OutputProtectionProxy)
an OutputProtectionDelegate with a null aura::Window* if the
RenderFrameHost is no longer alive while responding to a Mojo
request. Run failure callbacks in that case.

Bug: 987051
Test: None (sporadic crash with no repro steps)
Change-Id: I0b854aacdcdb553e349416cff76662d273d17e78
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1737367
Commit-Queue: Dominik Laskowski <domlaskowski@chromium.org>
Reviewed-by: Mitsuru Oshima <oshima@chromium.org>
Cr-Commit-Position: refs/heads/master@{#690073}

ash/display/output_protection_delegate.cc

@@ -33,7 +33,10 @@ OutputProtectionDelegate::OutputProtectionDelegate(aura::Window* window)
     : window_(window),
       display_id_(
           display::Screen::GetScreen()->GetDisplayNearestWindow(window).id()) {
-  DCHECK(window_);
+  // TODO(domlaskowski): OutputProtectionImpl passes null if the RenderFrameHost
+  // no longer exists. Investigate removing this check in crbug.com/997270.
+  if (!window_)
+    return;
 
   window_->AddObserver(this);
   display::Screen::GetScreen()->AddObserver(this);