Summary:
@public
Adds a class for triggering markers.
This calls `startMarker()` on construction, and `endMarker()` on destruction, thus being usable like a "scope guard": the object is instantiated, and automatically destroyed when going out of scope.
Reviewed By: SidharthGuglani
Differential Revision: D13817589
fbshipit-source-id: fd88884af970c1c0933d9ca6843f3f8f5d28b9e6
