When ANGLE is using the GL backend on GLX, we can expose
eglGetMscRateCHROMIUM via glXGetMscRateOML. Otherwise, this function
should return false.

Bug: chromium:1042393
Change-Id: Id9b308c2217e07ee9860e2869be0e23b7a0c7411
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2017048
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>