zero-copy: Clarify to allocate/destroy GpuMemoryBuffer on any thread and use it on the main thread o

ui/gl/gl_image_io_surface.h

Index: ui/gl/gl_image_io_surface.h
diff --git a/ui/gl/gl_image_io_surface.h b/ui/gl/gl_image_io_surface.h
index 8497278a058be0d5fb92899b21fc44e0005ca282..4ec85554c0fc74a98f7f84f47c2918f0c041feef 100644
--- a/ui/gl/gl_image_io_surface.h
+++ b/ui/gl/gl_image_io_surface.h
@@ -8,6 +8,7 @@
 #include <IOSurface/IOSurfaceAPI.h>
 
 #include "base/mac/scoped_cftyperef.h"
+#include "base/threading/thread_checker.h"
 #include "ui/gfx/gpu_memory_buffer.h"
 #include "ui/gl/gl_image.h"
 
@@ -41,6 +42,7 @@ class GL_EXPORT GLImageIOSurface : public GLImage {
  private:
   base::ScopedCFTypeRef<IOSurfaceRef> io_surface_;
   const gfx::Size size_;
+  base::ThreadChecker thread_checker_;
 
   DISALLOW_COPY_AND_ASSIGN(GLImageIOSurface);
 };