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}