This CL adds a MockAecDump and integration tests that inject the mock
into AudioProcessingImpl. The tests check the call pattern between
AudioProcessingImpl and AecDump. The existing tests ApmTest.* and
DebugDumpTest.* (not touched by this CL) check that the data written
by AecDumpImpl is valid.
The tests check that the protobuf-writing methods for the different
protobuf message types in audio_processing/debug.proto are indeed
called for the different modes of AudioProcessingImpl operation.
BUG=webrtc:7404
Review-Url: https://codereview.webrtc.org/2888533005
Cr-Commit-Position: refs/heads/master@{#18501}