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Create a single implementation of MotionEvent's |Clone()| and
|Cancel()| methods using MotionEventGeneric. Expand
MotionEventGeneric to support historical events, also removing
CompoundMotionEvent from the MotionEventBuffer implementation.

By avoiding JNI and garbage creation, this reduces the cost of both
methods by ~10x on Android, from ~25us call to ~2us.

BUG=501503003

Review URL: https://codereview.chromium.org/502993004

Cr-Commit-Position: refs/heads/master@{#301136}