Replace PreGrContextSubmit DCHECK with conditional

This check was added by
https://chromium-review.googlesource.com/c/chromium/src/+/2286052
and is failing for WebXR presentation when using immersive-ar on
Android.

Change-Id: Id47b7fb33ea9b89d5a782decde104c4ef5b4eccf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2288824
Commit-Queue: Klaus Weidner <klausw@chromium.org>
Commit-Queue: Peng Huang <penghuang@chromium.org>
Auto-Submit: Klaus Weidner <klausw@chromium.org>
Reviewed-by: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#786578}

components/viz/service/display_embedder/skia_output_device_buffer_queue.cc

@@ -121,8 +121,9 @@ void SkiaOutputDeviceBufferQueue::ScheduleOverlays(
 }
 
 void SkiaOutputDeviceBufferQueue::PreGrContextSubmit() {
-  DCHECK(current_image_);
-  current_image_->PreGrContextSubmit();
+  // The current image may be missing, for example during WebXR presentation.
+  if (current_image_)
+    current_image_->PreGrContextSubmit();
 }
 
 void SkiaOutputDeviceBufferQueue::SwapBuffers(