Fix null-dereference in X11SoftwareBitmapPresenter::FlushAfterPutImage

|event_task_runner_| may be nullptr when the browser is passed both
--single-process and --disable-gpu, so a null check is needed.

R=msisov

Change-Id: Ic045be1a70dfa98f269f47df7b9ccbb553a181d7
Bug: 1042170
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2004073
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Auto-Submit: Thomas Anderson <thomasanderson@chromium.org>
Reviewed-by: Maksim Sisov <msisov@igalia.com>
Cr-Commit-Position: refs/heads/master@{#732473}

ui/base/x/x11_software_bitmap_presenter.cc

@@ -185,7 +185,8 @@ void X11SoftwareBitmapPresenter::FlushAfterPutImage() {
   // blocking since there's no more data to read, so poll() won't wake up until
   // a new event comes, which may take a long time.  Forcing the event loop to
   // wake up with a dummy event fixes the race condition.
-  event_task_runner_->PostTask(FROM_HERE, base::BindOnce([] {}));
+  if (event_task_runner_)
+    event_task_runner_->PostTask(FROM_HERE, base::BindOnce([] {}));
 }
 
 void X11SoftwareBitmapPresenter::Resize(const gfx::Size& pixel_size) {