Add stable id to PaintImage.

cc/paint/paint_image.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_PAINT_IMAGE_H_
 #define CC_PAINT_PAINT_IMAGE_H_
 
 #include "base/logging.h"
 #include "cc/paint/paint_export.h"
 #include "third_party/skia/include/core/SkImage.h"
 
 namespace cc {
 
 // TODO(vmpstr): Add a persistent id to the paint image.
 class CC_PAINT_EXPORT PaintImage {
  public:
+  using Id = int;
+
+  // An id that can be used for all non-lazy images. Note that if an image is
+  // not lazy, it does not mean that this id must be used; one can still use
+  // GetNextId to generate a stable id for such images.
+  static const Id kNonLazyStableId = -1;
+
   // TODO(vmpstr): Work towards removing "UNKNOWN" value.
   enum class AnimationType { UNKNOWN, ANIMATED, VIDEO, STATIC };
 
   // TODO(vmpstr): Work towards removing "UNKNOWN" value.
   enum class CompletionState { UNKNOWN, DONE, PARTIALLY_DONE };
 
+  static Id GetNextId();
+
   PaintImage();
-  explicit PaintImage(sk_sp<SkImage> sk_image,
+  explicit PaintImage(Id id,
+                      sk_sp<SkImage> sk_image,
                       AnimationType animation_type = AnimationType::STATIC,
                       CompletionState completion_state = CompletionState::DONE);
   PaintImage(const PaintImage& other);
   PaintImage(PaintImage&& other);
   ~PaintImage();
 
   PaintImage& operator=(const PaintImage& other);
   PaintImage& operator=(PaintImage&& other);
 
   bool operator==(const PaintImage& other);
   explicit operator bool() const { return sk_image_; }
 
+  Id stable_id() const { return id_; }
   const sk_sp<SkImage>& sk_image() const { return sk_image_; }
   AnimationType animation_type() const { return animation_type_; }
   CompletionState completion_state() const { return completion_state_; }
 
  private:
+  Id id_ = kNonLazyStableId;
   sk_sp<SkImage> sk_image_;
   AnimationType animation_type_ = AnimationType::UNKNOWN;
   CompletionState completion_state_ = CompletionState::UNKNOWN;
 };
 
 }  // namespace cc
 
 #endif  // CC_PAINT_PAINT_IMAGE_H_