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}
2 files changed