cheets: implement cros side of WallpaperManagerService.

components/arc/common/wallpaper.mojom

Index: components/arc/common/wallpaper.mojom
diff --git a/components/arc/common/wallpaper.mojom b/components/arc/common/wallpaper.mojom
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index 0000000000000000000000000000000000000000..db66f14270910a57f843412667c9dac685a82769
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+++ b/components/arc/common/wallpaper.mojom
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+// Copyright 2016 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.
+//
+// Next MinVersion: 1
+
+module arc.mojom;
+
+// Handles wallpaper calls from ARC in Chrome.
+// Next method ID:2
+interface WallpaperHost {
+  // Gets current wallpaper encoded in PNG and send it back to ARC.
+  GetWallpaper@0() => (array<uint8> wallpaper);
+
+  // Sets an image from ARC as the wallpaper.
+  // |png_data| is a PNG encoded wallpaper image.
+  SetWallpaper@1(array<uint8> png_data);

[hidehiko, 2016/09/02 05:07:13]

Clarification: I'd like to know the clean-up plan.

So the plan is merging IntentHelper's SetWallpaper to here. IntentHelper's one
supports JPEG and this supports PNG. Who'll convert the image? I thought there
could be several approaches;
1) Convert image format in ARC container (JPEG -> PNG). Then I'm fine with this
API.
2) Pass through the image as is via Mojo, and in ArcWallpaperService, decode
images based on the format. Then, it's better to add file format to the API. Or,
alternatively you may be able to detect the file format by, e.g., magic.
3) Decode image in ARC container (JPEG -> Bitmap, PNG -> Bitmap). Then the file
format should be bitmap, here. The pros is simplicity, but the cons is that the
data will be bigger.

WDYT?

[Luis Héctor Chávez, 2016/09/02 16:23:48]

The intention is to drop the IntentHelper's SetWallpaper altogether, so nobody
will have to do JPEG encoding/decoding in the future :) Android's implementation
of the IntentHelper will call the WallpaperManager directly instead of having a
second way of setting the wallpaper.

[hidehiko, 2016/09/06 05:19:58]

Makes sense. Thank you for clarification.

+};
+
+// Connects with container side to publish wallpaper related intents.
+// Next method ID:1
+interface WallpaperInstance {
+  // Establishes communication with the container side.
+  Init@0(WallpaperHost host_ptr);
+};