Supports querying the CGLContextObj and CGLPixelFormatObj associated
with ANGLE's underlying OpenGL context on macOS.

Minor refactorings to implementation of device attribute queries on
all platforms.

Bug: angleproject:3973
Change-Id: I24341668be4cbfed0b7f2df4c1402df1effe275e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1846733
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>