Currently the TRACE_EVENT{0|1|2} macros in libwebrtc are plumbed to the gecko profiler. But several macro callers (those suffixed 1 and 2) provide detailed information that can be particularly handy when debugging. The existing integration misses these details as it only captures the name of the trace event.

I'm gonna guess there was no existing use of the tuple
serialization/deserialization code, because Bytes doesn't exist in the
deserializer, and cannot possibly function properly on tuple members that
serialize to a non-constant size, since it's called on a default-constructed
tuple.

This patch took inspiration in the deserializer for Variant and seems to work
fine.

This patch adds a part of TraceEvent.h that we had previously skipped. It
handles the conversion from the arg-ful TRACE_EVENTn forms to the internal
representation which carries around a dynamic number of args (num_args and
friends).

This allows us to hook into GeckoTraceEvent.h at a higher level, where all trace
events end up, with or without args.

This patch changes where we hook into GeckoTraceEvent.h and lets us capture
args.

Many revisions later... this is my best shoehorning attempt of webrtc's trace
events into the profiler.

It is not an optimal fit as the profiler wants a static schema, and the trace
event framework provides labels and values for the args dynamically.

But it allows displaying all args in the profiler which is a big step for
low-maintenance webrtc trace event integration into the profiler.

Backed out for causing failures at browser_interaction-between-interfaces.js.

Backout link: https://hg.mozilla.org/integration/autoland/rev/986ca78e8e52bd4620a3d2c794a60da16213bbde

Push with failures: https://treeherder.mozilla.org/jobs?repo=autoland&selectedTaskRun=D-i3nB2FQ7utXqeZylOaJg.0&resultStatus=testfailed%2Cbusted%2Cexception%2Cretry%2Cusercancel&revision=eccba94506f01f70acfba919fe6595392bbf753a

Failure log: https://treeherder.mozilla.org/logviewer?job_id=363081925&repo=autoland&lineNumber=4340

https://hg.mozilla.org/mozilla-central/rev/61aa46ad333a
https://hg.mozilla.org/mozilla-central/rev/686c81ffdf0e
https://hg.mozilla.org/mozilla-central/rev/ef3cab042f98
https://hg.mozilla.org/mozilla-central/rev/ff06d9e647c0