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If navigation isn't possible when GestureNavSimple::OnOverscrollModeChange is
called, we don't show the affordance. In this case, we shouldn't attempt to
navigate again when GestureNavSimple::OnOverscrollComplete is called (even
if we can).

BUG=NONE

Review-Url: https://codereview.chromium.org/2983893002
Cr-Commit-Position: refs/heads/master@{#488290}