Normalize delay-and-sum mask in Beamformer

This normalization is done in the Matlab Code but was never ported to the C++ version.

R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/37919004

Cr-Commit-Position: refs/heads/master@{#8279}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8279 4adac7df-926f-26a2-2b94-8c16560cd09d
3 files changed