Add stubs for AEC3 factory.

This CL creates empty placeholders for EchoCanceller3Factory. This
allows for moving the factory of AEC3 as soon as downstream has been
updated to include echo_canceller3_factory.h.

Bug: webrtc:8844
Change-Id: I77c53d8257291f189c637e1c9ed76c4e74be1858
Reviewed-on: https://webrtc-review.googlesource.com/53862
Commit-Queue: Gustaf Ullberg <gustaf@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22050}

api/BUILD.gn

@@ -224,8 +224,19 @@ rtc_source_set("aec3_config") {
   sources = [
     "audio/echo_canceller3_config.h",
   ]
+}
 
-  deps = []
+rtc_source_set("aec3_factory") {
+  visibility = [ "*" ]
+  sources = [
+    "audio/echo_canceller3_factory.cc",
+    "audio/echo_canceller3_factory.h",
+  ]
+
+  deps = [
+    ":aec3_config",
+    ":echo_control",
+  ]
 }
 
 rtc_source_set("echo_control") {
@@ -233,8 +244,6 @@ rtc_source_set("echo_control") {
   sources = [
     "audio/echo_control.h",
   ]
-
-  deps = []
 }
 
 rtc_source_set("audio_options_api") {

api/audio/echo_canceller3_factory.cc

@@ -0,0 +1,12 @@
+/*
+ *  Copyright (c) 2018 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+#include "api/audio/echo_canceller3_factory.h"
+
+// TODO(bugs.webrtc.org/8844): EchoCanceller3Factory will be moved here.

api/audio/echo_canceller3_factory.h

@@ -0,0 +1,19 @@
+/*
+ *  Copyright (c) 2018 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#ifndef API_AUDIO_ECHO_CANCELLER3_FACTORY_H_
+#define API_AUDIO_ECHO_CANCELLER3_FACTORY_H_
+
+#include "api/audio/echo_canceller3_config.h"
+#include "api/audio/echo_control.h"
+
+// TODO(bugs.webrtc.org/8844): EchoCanceller3Factory will be moved here.
+
+#endif  // API_AUDIO_ECHO_CANCELLER3_FACTORY_H_