cros: Fix video unittests on cros device.

Some ozone platforms (e.g. drm) expects to be able to watch a file
descriptor, so use a TYPE_IO message-loop for the gpu process host
thread.

BUG=715957
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.android:android_optional_gpu_tests_rel;master.tryserver.chromium.linux:linux_optional_gpu_tests_rel;master.tryserver.chromium.mac:mac_optional_gpu_tests_rel;master.tryserver.chromium.win:win_optional_gpu_tests_rel

Review-Url: https://codereview.chromium.org/2852553003
Cr-Commit-Position: refs/heads/master@{#467889}

media/gpu/video_decode_accelerator_unittest.cc

@@ -268,6 +268,10 @@ class VideoDecodeAcceleratorTestEnvironment : public ::testing::Environment {
     base::Thread::Options options;
 #if defined(OS_WIN)
     options.message_loop_type = base::MessageLoop::TYPE_UI;
+#elif defined(USE_OZONE)
+    // Some ozone platforms (e.g. drm) expects to be able to watch a file
+    // handler from this thread. So use the IO type message loop here.
+    options.message_loop_type = base::MessageLoop::TYPE_IO;
 #endif
     rendering_thread_.StartWithOptions(options);