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The fling curve for touchpad and touchscreen used to be configurable for
experimenting with, but these parameters have been stable for a long time now.
So make the values default, and remove all the code and preferences that were
necessary for the configurability.

BUG=none
R=avi@chromium.org, kenrb@chromium.org, sky@chromium.org, wjmaclean@chromium.org

Review URL: https://codereview.chromium.org/586933003

Cr-Commit-Position: refs/heads/master@{#296652}