On dual-GPU MacBook Pros, the maximum number of supported samples can
differ between the integrated and discrete GPUs, leading to unexpected
OpenGL errors in applications. Detected with WebGL conformance tests
in Safari with ANGLE as the WebGL backend.

Bug: angleproject:4518
Change-Id: I54997cbb83afd76d98b16d138d117d0afdeec5e1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2125407Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>