EGL: Implement ANGLE_metal_shared_event_sync on metal Implement creation of fence sync object taking an external MTLSharedEvent and optional value to use when signaling completion of prior commands. Extended end2end test suite with metal shared event test cases. Test: angle_end2end_tests --gtest_filter=EGLSyncTestMetalSharedEvent.* Bug: angleproject:7561 Change-Id: I0e72b5417275a20a24e535670ceb995ecc87abcb Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3865060 Reviewed-by: Quyen Le <lehoangquyen@chromium.org> Reviewed-by: Kenneth Russell <kbr@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>