cc: Audit and static_assert the types in PaintOpBuffer can be moved.

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 489cef27bd21647b902b0c85367b9f8447904dcd..97c4a28f19de5ee7a6cba7ff199c38f61c628944 100644
--- a/cc/paint/paint_image.h
+++ b/cc/paint/paint_image.h
@@ -55,6 +55,13 @@ class CC_PAINT_EXPORT PaintImage {
   CompletionState completion_state() const { return completion_state_; }
 
  private:
+  // Because PaintImage is stored in PaintOpBuffer it must be able to be moved
+  // in memory. This is satisfied since it has a default move constructor, and
+  // the same is satisfied by each of its members. But otherwise would need to
+  // ensure it directly or indirectly owns no pointers back to itself.
+  // sk_sp<SkImage> satisfies this since the SkImage does not contain a pointer
+  // to the sk_sp<> itself. The other types are trivial types.
+
   Id id_ = kUnknownStableId;
   sk_sp<SkImage> sk_image_;
   AnimationType animation_type_ = AnimationType::UNKNOWN;