minyue 84db6fa7f5 Adding BlockMeanCalculator for AEC.
This will improve the readability of AEC code.

BUG=

Review URL: https://codereview.webrtc.org/1805633006

Cr-Commit-Position: refs/heads/master@{#12123}
2016-03-24 21:36:33 +00:00
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