Bug 1716959 - Don't use lock from profiler_capture_backtrace_into - r=canaltinova

`profiler_capture_backtrace_into` now only uses thread-safe functions: ThreadRegistration::WithOnThreadRefOf, Register::SyncPopulate, DoSyncSample.
So we don't need to lock the main profiler mutex anymore.

This means that on-thread sampling (typically used in markers) can happen at the same time the periodic sampler has locked the profiler mutex and is sampling this and other threads.

Differential Revision: https://phabricator.services.mozilla.com/D122089

tools/profiler/core/platform.cpp

@@ -5509,8 +5509,6 @@ bool profiler_capture_backtrace_into(ProfileChunkedBuffer& aChunkedBuffer,
                                      StackCaptureOptions aCaptureOptions) {
   MOZ_RELEASE_ASSERT(CorePS::Exists());
 
-  PSAutoLock lock;
-
   if (!profiler_is_active() ||
       aCaptureOptions == StackCaptureOptions::NoStack) {
     return false;