gfx::Image: Added thread checker.

ui/gfx/image/image.h

Index: ui/gfx/image/image.h
diff --git a/ui/gfx/image/image.h b/ui/gfx/image/image.h
index 01344242d939a20cb9c6f7ca682104bee1d90d69..f5feeb22c6e8e824a096c3e9fcdb46849dafcf18 100644
--- a/ui/gfx/image/image.h
+++ b/ui/gfx/image/image.h
@@ -15,6 +15,18 @@
 //
 // Images can be empty, in which case they have no backing representation.
 // Attempting to use an empty Image will result in a crash.
+//
+// Image is *not thread-safe* for reads or writes. In Debug builds, a thread
+// checker will check that Images are not being used across multiple threads. If
+// you need to pass an image from one thread to another, you have two options:
+// 1. Ensure that you are the sole reference holder of the Image (keeping in
+//    mind that if the Image has been copied that the place you copied it from
+//    will hold a reference to it), and call DetachFromThread() just before
+//    passing the Image to another thread.
+// 2. Get the underlying image representation (e.g., ImageSkia) out of the
+//    image, ensure that it is thread-safe (ImageSkia requires special attention
+//    to make it thread-safe), pass that to another thread, then create a new
+//    Image object on the other side.
 
 #ifndef UI_GFX_IMAGE_IMAGE_H_
 #define UI_GFX_IMAGE_IMAGE_H_
@@ -179,6 +191,18 @@ class GFX_EXPORT Image {
   void SetSourceColorSpace(CGColorSpaceRef color_space);
 #endif  // defined(OS_MACOSX) && !defined(OS_IOS)
 
+  // Turns off thread checking on this Image (and all Images sharing its backing
+  // store). This should be used to mark code that is not currently thread-safe
+  // and any usage of this method should be considered a bug and accompanied by
+  // a TODO.
+  // TODO(mgiuca): Remove this method after all uses of Image are thread-safe.
+  void DisableThreadChecking();
+
+  // Informs the thread checker that this Image is about to be moved to another
+  // thread. Should only be used when this Image is the sole reference holder of
+  // its backing store.
+  void DetachFromThread();
+
  private:
   // Returns the type of the default representation.
   RepresentationType DefaultRepresentationType() const;