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/+/1987278Reviewed-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>