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Because for each overscroll session, OverscrollStarted() and
OverscrollCompleted() (in TouchEditableImplAura) are not necessarily
called the same number of times, TouchEditableImplAura could be stuck in
a state of thinking that there is always an scroll in progress which
prevents handles from appearing after a long-press.

BUG=430176

Review URL: https://codereview.chromium.org/963103003

Cr-Commit-Position: refs/heads/master@{#319761}