TracedValue: add a comment about rvalues.
TracedValue: add a comment about rvalues.

Based on feedback from dcheng@, add a brief comment explaining why
TraceValue's Write methods are rvalue-qualified.

R=skyostil@google.com

Change-Id: Ic22b6ed49b078b634467ea0d26925d8a4c97a1ed
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto