Remove legacy weirdness in Merge::Downsample
Remove legacy weirdness in Merge::Downsample

In practice, this will have only marginal effect. The length_limit
was increased from 6.7 ms to 10 ms. This is compared with the
input_length, which is equal to the decoded frame size. Thus,
this change will only affect encoded frame sizes in this range
(including 10 ms).

BUG=2696
R=tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/9969004

Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 367000fa8d4da2425c80c9bcf470d4f53ed39632

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