Add stable id to PaintImage.

cc/paint/paint_image.cc

Index: cc/paint/paint_image.cc
diff --git a/cc/paint/paint_image.cc b/cc/paint/paint_image.cc
index 846b3beecf6bc17923a6ba1280efc8b2e10e73f3..0a19352355c2c7ac979d6cf59ba9c249cab15f02 100644
--- a/cc/paint/paint_image.cc
+++ b/cc/paint/paint_image.cc
@@ -6,11 +6,15 @@
 
 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;
@@ -21,9 +25,13 @@ 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