Move Windows-specific code out of Fence.cpp. Split FenceImpl
based on suggestions on previous review
https://chromium-review.googlesource.com/221805/ . Refactored
further based on code review feedback and added first unit tests.

BUG=angleproject:774

Change-Id: I630034e1788e48ddb7722016ca22da474e785798
Reviewed-on: https://chromium-review.googlesource.com/222954Reviewed-by: Kenneth Russell <kbr@chromium.org>
Tested-by: Kenneth Russell <kbr@chromium.org>