The EGL_KHR_fence_sync spec says that if a flush is needed before waiting
on the sync, it's done on the current context for the current thread.

This helps simplify the multithreading design, we don't need to worry about
flusing on a context that may no longer exist or is executing on a different
thread.  It does allow infinite waits because the context with the fence is
never flushed but the spec allows this.

BUG=angleproject:2464

Change-Id: I8bf2f93c408fee2dae95caa5bb9c76ba67687931
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1542256Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>