Add stable id to PaintImage.

cc/paint/paint_image.cc

 // 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.
 
 #include "cc/paint/paint_image.h"
 
 namespace cc {
 
+base::AtomicSequenceNumber PaintImage::s_next_id_;
+
 PaintImage::PaintImage() = default;
-PaintImage::PaintImage(sk_sp<SkImage> sk_image,
+PaintImage::PaintImage(Id id,
+                       sk_sp<SkImage> sk_image,
                        AnimationType animation_type,
                        CompletionState completion_state)
-    : sk_image_(std::move(sk_image)),
+    : id_(id),
+      sk_image_(std::move(sk_image)),
       animation_type_(animation_type),
       completion_state_(completion_state) {}
 PaintImage::PaintImage(const PaintImage& other) = default;
 PaintImage::PaintImage(PaintImage&& other) = default;
 PaintImage::~PaintImage() = default;
 
 PaintImage& PaintImage::operator=(const PaintImage& other) = default;
 PaintImage& PaintImage::operator=(PaintImage&& other) = default;
 
 bool PaintImage::operator==(const PaintImage& other) {
-  return sk_image_ == other.sk_image_ &&
+  return id_ == other.id_ && sk_image_ == other.sk_image_ &&
          animation_type_ == other.animation_type_ &&
          completion_state_ == other.completion_state_;
 }
 
+PaintImage::Id PaintImage::GetNextId() {
+  return s_next_id_.GetNext();
+}
+
 }  // namespace cc