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When TouchEvents are copied the base::NativeEvent of the copy is set as null for
GenericEvent types, such as touchs.

Touch Release and Cancel events are set to remove a native touch id mapping upon
destruction.

This causes copied Touch Release/Cancel events to attempt to remove a mapping on
a null native event. Leading to a crash.

I've explicitly defined a copy constructor on TouchEvent to stop copied events
from attempting to remove this mapping.

TEST=EventsXTest.CopiedTouchEventNotRemovingFromNativeMapping
BUG=467102

Review URL: https://codereview.chromium.org/1026573002

Cr-Commit-Position: refs/heads/master@{#321563}