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Right now, Gesture Navigation has two very distinct phases: the initial
one, where the web contents window receives events and is moved, and the
follow up navigation, where the page still hasn't loaded and the user
scrolls on the overlay. The meat of this patch is to unify as much of
the logic as possible without sacrificing features. A new class is
introduced, OverscrollWindowAnimation, and the responsibility of the
existing OverscrollNavigationOverlay is steered into performing the
actual navigation.

TEST=Overscroll*
BUG=467692,464532,420121

Review URL: https://codereview.chromium.org/895543005

Cr-Commit-Position: refs/heads/master@{#324275}