Merge two files with AudioEncoderOpus tests

Merge two files with AudioEncoderOpus tests

Merge the contents of audio_encoder_mutable_opus_test.cc into
audio_encoder_opus_unittest.cc, since they're now both testing
AudioEncoderOpus.

(While preparing this CL, I noted a bug in the PacketLossRateOptimized
test. This CL leaves that test essentially unchanged; I've posted bug
4981 about the problem.)

Committed: https://crrev.com/77d22fa014e6ce8e315c9703b113f0422ab07de1
Cr-Commit-Position: refs/heads/master@{#9906}

[kwiberg-webrtc, 2015-09-09 08:56:50]

kwiberg@webrtc.org changed reviewers:
+ henrik.lundin@webrtc.org

[hlundin-webrtc, 2015-09-09 11:07:36]

lgtm

[kwiberg-webrtc, 2015-09-09 11:08:30]

The CQ bit was checked by kwiberg@webrtc.org

[commit-bot: I haz the power, 2015-09-09 11:08:37]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1319713004/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1319713004/20001

[commit-bot: I haz the power, 2015-09-09 11:38:38]

Committed patchset #2 (id:20001)

[commit-bot: I haz the power, 2015-09-09 11:38:49]

Patchset 2 (id:??) landed as
https://crrev.com/77d22fa014e6ce8e315c9703b113f0422ab07de1
Cr-Commit-Position: refs/heads/master@{#9906}