Support CHROMIUM_framebuffer_mixed_samples in the GL backend
BUG=angleproject:1378
Change-Id: I634c41f3507dbdeaa56234c4de1b498f81c848ed
Reviewed-on: https://chromium-review.googlesource.com/344520
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Sami Väisänen <svaisanen@nvidia.com>
