Upstream WebKit's iOS port of ANGLE. Added the EAGL backend authored by Dean Jackson from Apple, and the refactoring changes needed to support it side-by-side with the macOS backend. Ran "git cl format" against these diffs. Defined the EGL_ANGLE_device_eagl extension and allocated an enum out of ANGLE's reserved range. The iOS backend is not yet included in any of the GN files. Bug: angleproject:4263 Change-Id: I631c32930433c03bb16a242955ffedf55174bb29 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1987278 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com> Reviewed-by: James Darpinian <jdarpinian@chromium.org> Commit-Queue: Kenneth Russell <kbr@chromium.org>