Make PointerEvent an experimental web platform feature.

third_party/WebKit/LayoutTests/virtual/pointerevent/fast/events/pointerevents/mouse-pointer-transition-events-expected.txt

-Verifies that pointerenter/leave/over/out are fired correctly for correponding mouse events.
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-On success, you will see a series of "PASS" messages, followed by "TEST COMPLETE".
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-green received pointerover, at phase=target
-grey received pointerover, at phase=bubble
-grey received pointerenter, at phase=target
-green received pointerenter, at phase=target
-green received mouseover, at phase=target
-grey received mouseover, at phase=bubble
-grey received mouseenter, at phase=target
-green received mouseenter, at phase=target
-green received pointerout, at phase=target
-grey received pointerout, at phase=bubble
-green received pointerleave, at phase=target
-grey received pointerleave, at phase=target
-green received mouseout, at phase=target
-grey received mouseout, at phase=bubble
-green received mouseleave, at phase=target
-grey received mouseleave, at phase=target
-PASS successfullyParsed is true
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-TEST COMPLETE
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