[MBI] Recreate AgentSchedulingGroup when process changes.

Although the AgentSchedulingGroupHosts's RenderProcessHost remains
constant, the underlying process behind it might die and be restarted.
When that happens, we need to create a new AgentSchedulingGroup in the
new renderer process, and re-bind the mojo endpoints.
This is done by resetting the mojo endpoints when the process exits, and
having the various public methods rebind them when necessary.

Bug: 1111231
Change-Id: I48169dad231b5912078ff59f7bf6a370017d2031
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2397057


Commit-Queue: Tal Pressman <talp@chromium.org>
Reviewed-by: Minoru Chikamune <chikamune@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811082}