media/gpu/V4L2VDA: Fix sequence check failing in device poller on shutdown.

This CL fixes a sequence check failing when destroying the device
poller. This can happen when a video decoder is destroyed before
initialization, causing destruction to not be performed on the video
decoder thread as it was never started.

TEST=./video_decode_accelerator_tests on hana

BUG=b:150650090

Change-Id: I3f014c70cc5e7ffdd54d706a93ad12e014189a88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2084672
Commit-Queue: David Staessens <dstaessens@chromium.org>
Reviewed-by: Alexandre Courbot <acourbot@chromium.org>
Cr-Commit-Position: refs/heads/master@{#748156}

media/gpu/v4l2/v4l2_device_poller.cc

@@ -25,9 +25,12 @@ V4L2DevicePoller::V4L2DevicePoller(V4L2Device* const device,
 }
 
 V4L2DevicePoller::~V4L2DevicePoller() {
-  DCHECK_CALLED_ON_VALID_SEQUENCE(client_sequence_checker_);
-
-  StopPolling();
+  // It's possible the V4L2 device poller gets destroyed on a different thread
+  // than expected if e.g. destroying a decoder immediately after creation. The
+  // check here is not thread-safe, but using a lock or atomic state doesn't
+  // make sense as destruction is never thread-safe.
+  if (poll_thread_.IsRunning())
+    StopPolling();
 }
 
 bool V4L2DevicePoller::StartPolling(EventCallback event_callback,