Bug 1598992 - Stop Base Profiler before starting Gecko Profiler - r=gregtatum

This ensures that no more Base Profiler (BP) activity can happen when Gecko
Profiler (GP) starts.
In particular on Linux this is needed because the BP sampler sends signals to
stop threads, and the just-starting GP could receive this signal before it is
fully ready to handle it with its own sampler.

Differential Revision: https://phabricator.services.mozilla.com/D54443

--HG--
extra : moz-landing-system : lando