When |OS_FUCHSIA| is defined (even when |FUCHSIA_SDK| is defined as
well), use the Fuchsia SDK trace macros rather than the Dart timeline.

Reasons for doing this include:

Fuchsia's trace macros support categories.  This allows one to
distinguish between (e.g.) "flutter" and "skia" trace events for trace
recording and trace visualization.

Fuchsia has existing in tree benchmarks that depend on finding certain
events under category "flutter".

See the Fuchsia performance mailing list discussion for more context.