Revert of Add stable id to PaintImage.

cc/paint/discardable_image_store.h

Index: cc/paint/discardable_image_store.h
diff --git a/cc/paint/discardable_image_store.h b/cc/paint/discardable_image_store.h
index 426785c529a5acccf814c36d123a2d0212f9b049..1c26e79e57653e22e29435d7492ba40f4fceb81c 100644
--- a/cc/paint/discardable_image_store.h
+++ b/cc/paint/discardable_image_store.h
@@ -44,13 +44,6 @@
   std::unique_ptr<PaintTrackingCanvas> canvas_;
   std::vector<std::pair<DrawImage, gfx::Rect>>* image_set_;
   base::flat_map<ImageId, gfx::Rect>* image_id_to_rect_;
-  // This is currently used for images that come from shaders. We don't know
-  // what the stable id is, but since the completion and animation states are
-  // both unknown, this value doesn't matter as it won't be used in checker
-  // imaging anyway. Keep this value the same to avoid id churn.
-  // TODO(vmpstr): Remove this when we can add paint images into shaders
-  // directly.
-  PaintImage::Id unknown_stable_id_;
 };
 
 }  // namespace cc