Make TracedDictionary/Array implicitly constructible from TracedValue.

Allow TracedDictionary & TracedArray to be implicitly constructed from
TracedValue, similarly to TracedProto<T>.

This will allow to simplify a common case of writing a nested dictionary
/ array from

TRACE_EVENT("cat", "E", "data", [](perfetto::TracedValue ctx) {
  perfetto::TracedDictionary dict = std::move(ctx).WriteDictionary();
  dict.Add(...);
});

to

TRACE_EVENT("cat", "E", "data", [](perfetto::TracedDictionary dict) {
  dict.Add(...);
});

R=eseckler@google.com,skyostil@google.com
CC=​​primiano@google.com

Bug: b/184558843
Change-Id: If5391ff3f22654a307a04a8faa1b69cc21ecbdaa