eglGetMscRateCHROMIUM was added to EGL_CHROMIUM_sync_control based
on the original extension GLX_OML_sync_control. However, this new
function is not universally implemented. This CL moves it to a new
extension, EGL_ANGLE_sync_control_rate, and renames it to
eglGetMscRateANGLE.

Bug: chromium:1064078
Change-Id: Ia2a29c6776b2b2bf2b98e58ee83b5f141ed01301
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2118154
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Tim Van Patten <timvp@google.com>