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This CL adds elasticity to the simple gesture nav UI. After it reaches
the activation threshold, the affordance would be in its final shape
(full opacity, centered arrow, full ripple size), however it would still
move with further drags with diminishing speed until it completely
stops. Further drags would be ignored.

BUG=668296
TEST=manual

Change-Id: I7423e441bf90c04b84fda2cf9ddde44745bc945e
Reviewed-on: https://chromium-review.googlesource.com/559592
Commit-Queue: Mohsen Izadi <mohsen@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488840}