Add stable id to PaintImage.

cc/paint/discardable_image_store.h

 // Copyright 2017 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CC_PAINT_DISCARDABLE_IMAGE_STORE_H_
 #define CC_PAINT_DISCARDABLE_IMAGE_STORE_H_
 
 #include "base/containers/flat_map.h"
 #include "cc/paint/draw_image.h"
 #include "cc/paint/image_id.h"
@@ skipping 26 matching lines @@
                 const SkRect& src_rect,
                 const SkRect& rect,
                 const SkMatrix* local_matrix,
                 const PaintFlags& flags);
 
   // This canvas is used only for tracking transform/clip/filter state from the
   // non-drawing ops.
   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
 
 #endif  // CC_PAINT_DISCARDABLE_IMAGE_STORE_H_