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As the samples' channels should be ordered (accel_x, accel_y, accel_z),
using ++it should reduce the complexity slightly compared with using
find for each channel.

BUG=b:172208566, b:172414227
TEST=builds, unit tests, and run on octopus

Change-Id: Ibfcb385f71aec5c202a50940c04954393be0817a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2519348
Commit-Queue: Cheng-Hao Yang <chenghaoyang@chromium.org>
Reviewed-by: Xiaoqian Dai <xdai@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828055}