Tracing: Remove the Coordinator::StartTracing callback

This callback was previously called when all of the
currently connected tracing agents had ACKed the StartTracing
call, plus any new agents that connected while the controller
was still waiting for any existing agents to ACK. Since agents
can dynamically be added/removed at any time, this particular
callback is not particularly useful; in practice it only got used
for one purpose: For tests and devtools to know when they could
start emitting trace events that'll make it into the trace, after
a StartTracing call.

This CL removes the callback from the tracing::Coordinator mojo
interface, and instead adds a TraceLog observer to the
TracingCoordinatorImpl that lets us more reliably and directly
let tracing clients know when they can start emitting events.

Perfetto doesn't currently have a concept of
"let me know when the Consumers are notified to begin
tracing". so this saves some future implementation work in order to make
the switch.

TBR=yusukes@chromium.org
Bug: 914579

Change-Id: I3fc4bf4a06b93938e053bb13ab098881c485b239
Reviewed-on: https://chromium-review.googlesource.com/c/1439082
Commit-Queue: oysteine <oysteine@chromium.org>
Reviewed-by: Nasko Oskov <nasko@chromium.org>
Reviewed-by: Eric Seckler <eseckler@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#627423}(cherry picked from commit 3d8e0ad7f87d2c3b91cfe528ab9948ce4299a3dd)
Reviewed-on: https://chromium-review.googlesource.com/c/1452365
Reviewed-by: oysteine <oysteine@chromium.org>
Cr-Commit-Position: refs/branch-heads/3683@{#165}
Cr-Branched-From: e51029943e0a38dd794b73caaf6373d5496ae783-refs/heads/master@{#625896}
25 files changed