Introduce ash mojo interface for lock screen action handlers

ash/public/interfaces/action_handler.mojom

Index: ash/public/interfaces/action_handler.mojom
diff --git a/ash/public/interfaces/action_handler.mojom b/ash/public/interfaces/action_handler.mojom
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+// 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 ash.mojom;
+
+// Supported actions.
+enum ActionHandlerAction { kNewNote, };
+
+// An action handler state.
+enum ActionHandlerState {
+  // The client does not support the action.
+  kNotSupported,
+  
+  // The client supports the action and is not currently handling the action.
+  kAvailable,
+
+  // The client received the request for the action and it is launching the
+  // flow to handle it.
+  kLaunching,
+
+  // The client is currently handling the action.
+  kActive,
+
+  // The client is currently handling the action, but the handler has been
+  // hidden (e.g. for lock screen action, the action handler is backgrounded by
+  // the lock UI).
+  kHidden,
+};
+
+// Used by a client (e.g. Chrome) to update state of app action handlers.
+interface ActionHandlerStateController {

[James Cook, 2017/05/02 15:43:38]

"ActionHandler" seems like a very generic name. Is the intent that this will be
used for all sorts of generic actions, or mostly lock screen and/or shelf
actions? If it's lock screen/shelf the name should reflect that.

Also, our usual naming pattern is:

FooController (in ash)
FooControllerClient or FooClient (in chrome)

[tbarzic, 2017/05/02 17:05:54]

Yeah, I'm not to content with the name either.

The intent is to use this with Chrome apps that support action handler API -
which currently supports a single action - new note.

Though, from ash perspective there is nothing that would stop ash to use this
interface for more generic actions.

I guess ShelfActionHandler might be slightly better naming.

Other option I considered was AppActionHandler, though "app" seems more like
Chrome than ash concept (and the fact these actions are handled by chrome/arc
apps seems like the client implementation detail).

+   // Sets the action handler to be used to handle action requests.
+   SetActionHandler(ActionHandler action_handler);
+
+  // Sets state of an app action available on lock screen.
+  UpdateActionState(ActionHandlerAction action, ActionHandlerState state);
+};
+
+// Used by ash to request Chrome to handle an action.
+interface ActionHandler {
+
+  // Requests an action to be handled.
+  RequestHandleAction(ActionHandlerAction action);
+};