Introduce BeginFrameSource::SchedulerClient

Currently BeginFrameObservers are notified in the order in which they
begin observation. This means there is no guarantee that
viz::DisplayScheduler will be notified after all clients. Since the
OnBeginFrame signal is used to make Deadline decisions, we can end up
choosing the incorrect deadline based on partial information.

This change adds an alternative way to observer the BeginFrames. The
BeginFrameSource::SchedulerClient will always be notified after all
other observers. We have viz::DisplayScheduler implement this.

This is controlled behind a flag, so we aren't removing the old observer
path for now

Bug: 40900977

Change-Id: Ic62c522e6920b05a35388fdeea09f8717730801b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6998406
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1524466}