Switch DRM platform to using a separate thread

ui/ozone/platform/drm/gpu/drm_device_manager.h

Index: ui/ozone/platform/drm/gpu/drm_device_manager.h
diff --git a/ui/ozone/platform/drm/gpu/drm_device_manager.h b/ui/ozone/platform/drm/gpu/drm_device_manager.h
index 34fe61335d62cb1bb9165d8ef3d7ea0467130f68..40991df0a97188f1115b14b32cb8e0abf7221de5 100644
--- a/ui/ozone/platform/drm/gpu/drm_device_manager.h
+++ b/ui/ozone/platform/drm/gpu/drm_device_manager.h
@@ -18,7 +18,6 @@
 namespace base {
 class FilePath;
 struct FileDescriptor;
-class SingleThreadTaskRunner;
 }
 
 namespace ui {
@@ -41,9 +40,6 @@ class OZONE_EXPORT DrmDeviceManager {
   bool AddDrmDevice(const base::FilePath& path, const base::FileDescriptor& fd);
   void RemoveDrmDevice(const base::FilePath& path);
 
-  void InitializeIOTaskRunner(
-      const scoped_refptr<base::SingleThreadTaskRunner>& task_runner);
-
   // Updates the device associated with |widget|.
   void UpdateDrmDevice(gfx::AcceleratedWidget widget,
                        const scoped_refptr<DrmDevice>& device);
@@ -64,8 +60,6 @@ class OZONE_EXPORT DrmDeviceManager {
 
   scoped_ptr<DrmDeviceGenerator> drm_device_generator_;
 
-  scoped_refptr<base::SingleThreadTaskRunner> io_task_runner_;
-
   DrmDeviceVector devices_;
 
   std::map<gfx::AcceleratedWidget, scoped_refptr<DrmDevice>> drm_device_map_;