Fix implicit channel layout configurations at non-1.0 playback rates.

Change http://crrev.com/21d17ca06e742cbfdcaaa54e0f4964248e3fbfff
incorrectly assumed the input channel layout never changes. This
patch fixes that issue by allowing the channel mask to change as
layouts change.

BUG=666813
TEST=updated unittests
TEST=http://storage.googleapis.com/chcunningham-chrome-shared/534301/aac_test.html

Review-Url: https://codereview.chromium.org/2536013002
Cr-Commit-Position: refs/heads/master@{#434868}
7 files changed