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/codecs/isac/unittest.cc b/webrtc/modules/audio_coding/codecs/isac/unittest.cc
index d05ffa6..673d290 100644
--- a/webrtc/modules/audio_coding/codecs/isac/unittest.cc
+++ b/webrtc/modules/audio_coding/codecs/isac/unittest.cc
@@ -111,7 +111,7 @@
   typename T::instance_type* encdec;
   ASSERT_EQ(0, T::Create(&encdec));
   ASSERT_EQ(0, T::EncoderInit(encdec, adaptive ? 0 : 1));
-  ASSERT_EQ(0, T::DecoderInit(encdec));
+  T::DecoderInit(encdec);
   ASSERT_EQ(0, T::SetEncSampRate(encdec, sample_rate_hz));
   if (adaptive)
     ASSERT_EQ(0, T::ControlBwe(encdec, bit_rate, frame_size_ms, false));
@@ -129,7 +129,7 @@
     ASSERT_EQ(0, T::Control(enc, bit_rate, frame_size_ms));
   typename T::instance_type* dec;
   ASSERT_EQ(0, T::Create(&dec));
-  ASSERT_EQ(0, T::DecoderInit(dec));
+  T::DecoderInit(dec);
   T::SetInitialBweBottleneck(dec, bit_rate);
   T::SetEncSampRateInDecoder(dec, sample_rate_hz);