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}
diff --git a/webrtc/modules/audio_coding/neteq/neteq_impl.cc b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
index 22e71f7..cf7afbc 100644
--- a/webrtc/modules/audio_coding/neteq/neteq_impl.cc
+++ b/webrtc/modules/audio_coding/neteq/neteq_impl.cc
@@ -1178,15 +1178,14 @@
 
   if (reset_decoder_) {
     // TODO(hlundin): Write test for this.
-    // Reset decoder.
-    if (decoder) {
-      decoder->Init();
-    }
+    if (decoder)
+      decoder->Reset();
+
     // Reset comfort noise decoder.
     AudioDecoder* cng_decoder = decoder_database_->GetActiveCngDecoder();
-    if (cng_decoder) {
-      cng_decoder->Init();
-    }
+    if (cng_decoder)
+      cng_decoder->Reset();
+
     reset_decoder_ = false;
   }
 
@@ -1896,11 +1895,9 @@
     mute_factor_array_[i] = 16384;  // 1.0 in Q14.
   }
 
-  // Reset comfort noise decoder, if there is one active.
   AudioDecoder* cng_decoder = decoder_database_->GetActiveCngDecoder();
-  if (cng_decoder) {
-    cng_decoder->Init();
-  }
+  if (cng_decoder)
+    cng_decoder->Reset();
 
   // Reinit post-decode VAD with new sample rate.
   assert(vad_.get());  // Cannot be NULL here.