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When running on Fuchsia, "fml/trace_event.h" sends trace events towards Fuchsia's tracing system, rather than its own |fml::tracing::*| one. Do the same thing for |FlutterEventTracer|, in order to unlock category "skia" trace events when running on Fuchsia. The reason this was not working before is because |FlutterEventTracer| was directly using |fml::tracing::*| functions, rather than the TRACE_EVENT_* macros. In the future, we can look into having the skia event tracer enabled by default on Fuchsia, controlling the presence of Skia events by enabling/disabling the "skia" category, once we measure the performance cost of doing so.