This CL adds code to the BlockProcessor, which basically constitutes
the second layer in echo canceller 3. The CL includes two incomplete
classes (EchoRemover and EchoPathDelayEstimator) which will be completed
in upcoming CLs. Because of this, some of the unittests are disabled
until those are added.
BUG=webrtc:6018
Review-Url: https://codereview.webrtc.org/2611223003
Cr-Commit-Position: refs/heads/master@{#16319}