Defer loading tasks when awaiting touch response

Defer loading tasks when awaiting touch response

Initial scroll response latency is highly sensitive to dispatch of
touchstart and the first touchmove events. Intervening operations
between the touchstart and touchmove can delay scroll event dispatch,
increasing real and perceived latency.

Introduce a touchstart-specific policy wherein loading tasks are
completely deferred. This mode is entered upon observation of a
touchstart event, and exited when either 1) actionable events like touch
movement or scroll updates are observed, or 2) a 100ms timeout expires.

BUG=452051,391005

Review URL: https://codereview.chromium.org/877143003

Cr-Commit-Position: refs/heads/master@{#315701}