New interface (AudioDecoderFactory), with an implementation

This is a first draft of what we're hoping to use to create all
AudioDecoder instances. Follow-up CLs will start using this internally
in NetEq instead of calling constructors manually.

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/1917163002
Cr-Commit-Position: refs/heads/master@{#12548}
diff --git a/webrtc/modules/modules.gyp b/webrtc/modules/modules.gyp
index 502454c..3b36208 100644
--- a/webrtc/modules/modules.gyp
+++ b/webrtc/modules/modules.gyp
@@ -133,6 +133,7 @@
             'audio_processing',
             'audioproc_test_utils',
             'bitrate_controller',
+            'builtin_audio_decoder_factory',
             'bwe_simulator',
             'cng',
             'isac_fix',
@@ -167,6 +168,7 @@
             '<(webrtc_root)/tools/tools.gyp:agc_test_utils',
           ],
           'sources': [
+            'audio_coding/codecs/audio_decoder_factory_unittest.cc',
             'audio_coding/codecs/cng/audio_encoder_cng_unittest.cc',
             'audio_coding/acm2/acm_receiver_unittest_oldapi.cc',
             'audio_coding/acm2/audio_coding_module_unittest_oldapi.cc',