Perfetto UI - d148402da33b40

Add a hook to get early access to trace object before loading (#169)

Perfetto plugins in host apps need to be able to access contextual
information associated with the trace during loading of the trace
and initialization of the plugin instance. This requires that a host app
be able to associate information with the trace before the AppImpl's
promise result from its various trace loading APIs is resolved at the
completion of loading. Accordingly, add an optinoal call-back function
parameter to the AppImpl load method implementations to provide this
trace early access hook.

For android-graphics/sokatoa#4633

Signed-off-by: Christian W. Damus <cdamus@eclipsesource.com>