ash: HiDPI user avatar for SessionController

ui/gfx/image/mojo/image_skia.mojom

+// 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.
+
+module gfx.mojom;
+
+// Mojo transport for ImageSkiaRep via shared buffer.

[msw, 2017/03/23 20:47:51]

I wonder if we should [add a todo/bug comment to] make the shared buffer work at
the SkBitmap level (and then maybe skip ImageSkiaRep, and let ImageSkia simply
be an array of SkBitmaps)?

[xiyuan, 2017/03/24 05:45:19]

I think that would be helpful
- Modified the CL to extract the shared buffer part out as SharedBufferSkBitmap
and map it to SkBitmap. 
- I left a TODO comment to move SharedBufferSkBitmap into skia and use shared
buffer more efficiently.

And we could use gfx.mojom.SharedBufferSkBitmap ATM to use SkBitmap with mojo
via shared buffer.

I want to keep mojom.ImageSkiaRep because it stores a scale factor. Storing the
bitmap with its corresponding scale together is a plus to me. Otherwise, we need
to put scale in mojom.ImageSkia and has to keep the order. And this way is more
similar to our C++ side classes.

+struct ImageSkiaRep {
+  // Shared buffer handle to holds a serialized SkBitmap.

[msw, 2017/03/23 20:47:51]

nit: "to hold" or "that holds"

[xiyuan, 2017/03/24 05:45:19]

Done.

+  handle<shared_buffer> shared_buffer_handle;

[msw, 2017/03/23 20:47:51]

I'm not well versed with shared memory patterns (mojo or otherwise). I don't
have any specific concerns, but you may wish to include an additional reviewer
to ensure we haven't missed anything.

[xiyuan, 2017/03/24 05:45:19]

Acknowledged. Will ask sadrul@ to take a look.

+
+  // Byte size of the shared buffer.
+  uint32 buffer_byte_size;
+
+  // Corresponding scale of the bitmap.
+  float scale;
+};
+
+// Mojo transport for existing representations of an ImageSkia via shared
+// buffer. Note it is caller's responsibility to ensure the ImageSkia contains

[msw, 2017/03/23 20:47:51]

nit: "Note that it is the caller's"

[xiyuan, 2017/03/24 05:45:20]

Update comment to reflect that we do MakeThreadSafe() in struct traits.

+// all needed representations.
+struct ImageSkia {
+  array<ImageSkiaRep> image_reps;
+};