Add a Display and ContextProvider concept to mojom, use to recreate

mojo/services/gpu/public/interfaces/context_provider.mojom

+// Copyright 2015 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 mojo;
+
+import "gpu/public/interfaces/command_buffer.mojom";
+import "gpu/public/interfaces/viewport_parameter_listener.mojom";
+
+// A ContextProvider can be used to provide new command buffers related to a
+// particular context, for instance configured to draw to a particular display.
+interface ContextProvider {
+  // This initializes a new command buffer for this provider when available,
+  // closing connections to the previously created command buffer (if any).
+  //
+  // This call may not return for an extended period of time if a command
+  // buffer cannot be produced immediately, for example if it is associated
+  // with a view that is offscreen.
+  //
+  // If a viewport_parameter_listener is provided it will notified be when
+  // vsync parameters change for this context.

[qsr, 2015/03/04 10:33:18]

Do you want to comment about when the returned buffer can be null? Given that we
wait for a context to be available, returning an null pointer will only happen
if the consumer is calling this while another request is still in progress.

[jamesr, 2015/03/04 23:00:27]

Done.

+  Create(ViewportParameterListener? viewport_parameter_listener)
+      => (CommandBuffer? gles2_client);
+};