MockAudioEncoder: Use a dedicated marker method for test expectations

This makes the sequence of expected calls easier to read. Also, we can
save one line and get rid of a gmock warning by expecting the
MockAudioEncoder object to be destroyed at the end of the test instead
of making a final marker call.

Review URL: https://codereview.webrtc.org/1331793003

Cr-Commit-Position: refs/heads/master@{#9916}

webrtc/modules/audio_coding/codecs/mock/mock_audio_encoder.h

@@ -21,6 +21,7 @@ class MockAudioEncoder final : public AudioEncoder {
  public:
   ~MockAudioEncoder() override { Die(); }
   MOCK_METHOD0(Die, void());
+  MOCK_METHOD1(Mark, void(std::string desc));
   MOCK_CONST_METHOD0(MaxEncodedBytes, size_t());
   MOCK_CONST_METHOD0(SampleRateHz, int());
   MOCK_CONST_METHOD0(NumChannels, int());