This profiler-controlling functions are used in very few places, so it's good to have them in a separate header to reduce dependencies.
On top of making GeckoProfiler.h lighter, this is actually needed for this bug, because a later patch adds MozPromise to profiler_start and others, which would have created a header loop (GeckoProfiler -> MozPromise -> Monitor -> CondVar -> GeckoProfiler) that makes the build fail.
Differential Revision: https://phabricator.services.mozilla.com/D139331
