Add stable id to PaintImage.

cc/paint/paint_image.h

Index: cc/paint/paint_image.h
diff --git a/cc/paint/paint_image.h b/cc/paint/paint_image.h
index a79bcf02ca3e71085c7cfc57084eb186a3d84443..5aec3cfcdf21402c7813de51a8227295c5465ee1 100644
--- a/cc/paint/paint_image.h
+++ b/cc/paint/paint_image.h
@@ -14,14 +14,24 @@ 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);
@@ -34,11 +44,13 @@ class CC_PAINT_EXPORT PaintImage {
   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;