AudioDecoder: Replace Init() with Reset()

The Init() method was previously used to initialize and reset
decoders, and returned an error code. The new Reset() method is used
for reset only; the constructor is now responsible for fully
initializing the AudioDecoder.

Reset() doesn't return an error code; it turned out that none of the
functions it ended up calling could actually fail, so this CL removes
their error return codes as well.

R=henrik.lundin@webrtc.org

Review URL: https://codereview.webrtc.org/1319683002 .

Cr-Commit-Position: refs/heads/master@{#9798}

webrtc/modules/audio_coding/main/test/opus_test.cc

diff --git a/webrtc/modules/audio_coding/main/test/opus_test.cc b/webrtc/modules/audio_coding/main/test/opus_test.cc
index 79124aa..d6482dd 100644
--- a/webrtc/modules/audio_coding/main/test/opus_test.cc
+++ b/webrtc/modules/audio_coding/main/test/opus_test.cc
@@ -84,8 +84,8 @@
   // Create Opus decoders for mono and stereo for stand-alone testing of Opus.
   ASSERT_GT(WebRtcOpus_DecoderCreate(&opus_mono_decoder_, 1), -1);
   ASSERT_GT(WebRtcOpus_DecoderCreate(&opus_stereo_decoder_, 2), -1);
-  ASSERT_GT(WebRtcOpus_DecoderInit(opus_mono_decoder_), -1);
-  ASSERT_GT(WebRtcOpus_DecoderInit(opus_stereo_decoder_), -1);
+  WebRtcOpus_DecoderInit(opus_mono_decoder_);
+  WebRtcOpus_DecoderInit(opus_stereo_decoder_);
 
   ASSERT_TRUE(acm_receiver_.get() != NULL);
   EXPECT_EQ(0, acm_receiver_->InitializeReceiver());