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The ABS_X and ABS_Y axes are not part of the MT protocol and should
therefore not be used to update MT slot positions.

The symptom of this was secondary touches warping to the position of the
first touch in the X and/or Y axis, since ABS_X/Y always corresponds to
the first touch.

BUG=407386
TEST=chrome on pixel with --ash-touch-hud
NOTRY=true

Review URL: https://codereview.chromium.org/506303002

Cr-Commit-Position: refs/heads/master@{#292263}