Refactor ozone drm cursor code for mojo

ui/ozone/platform/drm/host/drm_cursor.cc

Index: ui/ozone/platform/drm/host/drm_cursor.cc
diff --git a/ui/ozone/platform/drm/host/drm_cursor.cc b/ui/ozone/platform/drm/host/drm_cursor.cc
index 6ee48c8cfa06983c58c90ce306658022b96d6ac9..9f125594fb609a128203aaf69c657d1a187a1ae8 100644
--- a/ui/ozone/platform/drm/host/drm_cursor.cc
+++ b/ui/ozone/platform/drm/host/drm_cursor.cc
@@ -27,6 +27,7 @@ class NullProxy : public DrmCursorProxy {
                  const gfx::Point& point,
                  int frame_delay_ms) override {}
   void Move(gfx::AcceleratedWidget window, const gfx::Point& point) override {}
+  void InitializeOnEvdev() {}
 
  private:
   DISALLOW_COPY_AND_ASSIGN(NullProxy);
@@ -177,6 +178,7 @@ void DrmCursor::MoveCursorTo(const gfx::PointF& screen_location) {
 }
 
 void DrmCursor::MoveCursor(const gfx::Vector2dF& delta) {
+  DCHECK(evdev_thread_checker_.CalledOnValidThread());
   TRACE_EVENT0("drmcursor", "DrmCursor::MoveCursor");
   base::AutoLock lock(lock_);
 
@@ -210,6 +212,11 @@ gfx::Rect DrmCursor::GetCursorConfinedBounds() {
   return confined_bounds_ + display_bounds_in_screen_.OffsetFromOrigin();
 }
 
+void DrmCursor::InitializeOnEvdev() {
+  evdev_thread_checker_.DetachFromThread();

[dnicoara, 2016/06/21 14:14:16]

Unless this is not called on the evdev thread you shouldn't detach. (In fact you
should DCHECK it since that makes sure |evdev_tread_checker_| is attached to the
correct thread)

[rjkroege, 2016/06/21 17:20:52]

The evdev_thread_checker_ is initialized on the main thread in the constructor.
So, I need to call DetachFromThread here to force the thread checker to be
attached to the evdev thread instead yes? This is true empirically: the thread
where this class is constructed is most definitely not the same thread as where
InitializeOnEvdev is called.

[dnicoara, 2016/06/21 17:38:18]

Ah, I see your reasoning. My understanding is that you should then call
DetachFromThread() when |evdev_thread_checker_| is created (in DrmCursor
constructor) and call CalledOnValidThread() in here. That way all call sites
that need to be done on the Evdev thread are checked.

+  proxy_->InitializeOnEvdev();
+}
+
 void DrmCursor::SetCursorLocationLocked(const gfx::PointF& location) {
   gfx::PointF clamped_location = location;
   clamped_location.SetToMax(gfx::PointF(confined_bounds_.origin()));