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>
