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Each enqueued event in the browser-side touch queue maintains a vector
of events that must be ack'ed, allowing each enqueued event to coalesce
with subsequent events delivered from the platform. Currently, the
vector is always populated with the first event, even if no coalescing
occurs. Instead, defer population until coalescing occurs, optimizing
the common case for most events by avoiding an extra heap allocation for
every touch. This can save ~5-10us per forwarded touch event on a typical
Android device.

BUG=384562

Review URL: https://codereview.chromium.org/565033002

Cr-Commit-Position: refs/heads/master@{#295998}