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A valid scenario, observed in the wild, is a touchstart handler removing
itself and adding a touchmove handler. If those are the only touch
handlers on the document, the renderer will forward two distinct handler
existence notifications to the browser. This causes the TouchEventQueue
to effectively drop the remainder of the touch sequence, even though the
element targetted by the touchstart now has a touchmove.

Fix this scenario by making the TouchEventQueue effectively idempotent
to repeated addition and removal of touch handlers. In practice, this
means simplifying its statefulness and instead relying on the existing
pointer id state map to determine whether to forward touches for the
remainder of the sequence.

BUG=412723

Review URL: https://codereview.chromium.org/586553002

Cr-Commit-Position: refs/heads/master@{#296205}