Simplify mask calculation

There are only 2 things that prevent the output to be bit-exact:
* The zero initialization of the postfilter_mask_ and high_pass_postfilter_mask_, which only afects the first blocks.
* The re-tuning of the target presence estimation, since only the bins between low_average_start_bin_ and high_average_end_bin_ are of interest.
This latter was not taken into account before.

R=andrew@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/35139004

Cr-Commit-Position: refs/heads/master@{#8368}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8368 4adac7df-926f-26a2-2b94-8c16560cd09d
2 files changed