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

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"
@@ skipping 37 matching lines @@
 
   bool operator==(const PaintImage& other) const;
   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:
+  // Because PaintImage is stored in PaintOpBuffer it must be able to be moved

[danakj, 2017/05/19 20:35:35]

I iterated on this comment a couple times to get something accurate and
descriptive, should be good now.

+  // 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;
   CompletionState completion_state_ = CompletionState::UNKNOWN;
 };
 
 }  // namespace cc
 
 #endif  // CC_PAINT_PAINT_IMAGE_H_