tracing_agents: atrace: Stop trace before async dump
tracing_agents: atrace: Stop trace before async dump

For long traces, async_dump can take a long time. During this time the tracer
is still running. Stop the tracer before collecting the dump so that the traces
are properly generated.

Signed-off-by: Joel Fernandes <89f39a38232ac523c7644e47b6ca6563177e40b4@google.com>
Bug: catapult:#
Change-Id: I62edb94d1488c3636d48b8e97b7fa40497326e31
Reviewed-on: https://chromium-review.googlesource.com/688832
Reviewed-by: John Reck <71b30919c768f293772415a2a3deb09223257fb5@google.com>
Reviewed-by: Chris Craik <24a70e8781b9b8b74f2d02f7b3f73348969c172b@google.com>

catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult,catapult-project/catapult