Add Mojo interface for AudioDecoder

For Spitzer, the audio decoder that supports encryption has to be
instantiated in the GPU process. The decoder has to be split
between a proxy and a service.

This CL contains Mojo interface only.

BUG=542910

Committed: https://crrev.com/cabbc3cc9b0c18f25d91f17bb6cdded528366695
Cr-Commit-Position: refs/heads/master@{#381360}

Review URL: https://codereview.chromium.org/1809483002

Cr-Commit-Position: refs/heads/master@{#381544}

media/mojo/interfaces/BUILD.gn

@@ -6,6 +6,7 @@ import("//mojo/public/tools/bindings/mojom.gni")
 
 mojom("interfaces") {
   sources = [
+    "audio_decoder.mojom",
     "content_decryption_module.mojom",
     "decryptor.mojom",
     "demuxer_stream.mojom",

media/mojo/interfaces/audio_decoder.mojom

@@ -0,0 +1,43 @@
+// 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.
+
+module media.interfaces;
+
+import "media/mojo/interfaces/media_types.mojom";
+
+interface AudioDecoder {
+  // Status of a decode operation. See media::AudioDecoder for description.
+  enum DecodeStatus {
+    OK,            // We're all good.
+    ABORTED,       // We aborted as a result of Reset() or destruction.
+    DECODE_ERROR,  // A decoding error occurred.
+  };
+
+  // Initializes the AudioDecoder with the audio codec configuration and CDM id.
+  // For the unencrypted streams the |cdm_id| is ignored. Executed the callback
+  // with whether the initialization succeeded, and whether the pipeline needs
+  // bitstream conversion.
+  Initialize(AudioDecoderConfig config, int32 cdm_id)
+     => (bool success, bool needs_bitstream_conversion);
+
+  // Sends the |buffer| to the underlying codec. Should be called only after
+  // Initialize() succeeds. The callback with the status is called after the
+  // decoder has accepted corresponding DecoderBuffer, indicating that the
+  // pipeline can send next buffer to decode.
+  // If |buffer| is an EOS buffer then the decoder must be flushed, i.e. all
+  // pending buffers should be processed, the corresponding decoded buffers
+  // should be returned to the proxy, and only then the service should return
+  // DecoderStatus.
+  Decode(DecoderBuffer buffer) => (DecodeStatus status);
+
+  // Resets decoder state. Should be called only if Initialize() succeeds.
+  // All pending Decode() requests will be finished or aborted, then the method
+  // executes the callback.
+  Reset() => ();
+};
+
+interface AudioDecoderClient {
+  // Sends the decoded audio buffer back to the proxy.
+  OnBufferDecoded(AudioBuffer buffer);
+};