Allow typed proto messages to be written into TracedValue.

This patch adds TracedValue::WriteProto<proto::pbzero::MessageType>()
method, which can be used to write structured proto inside TracedValue.

Also TracedProto now gets an implicit constructor from TracedValue,
which means that if a C++ class implements
Foo::WriteIntoTrace(TracedProto<...>) method, it can be used in all
places where a class implementing Foo::WriteIntoTrace(TracedValue) can
be used, including:
- being passed as an untyped argument to TRACE_EVENT:
  TRACE_EVENT(..., "arg", Foo());
- being passed to TracedDictionary::Add / TracedArray::Append.

This patch also adds MessageType::GetName() method to the protozero
bindings, which is needed for implementation of WriteProto method.

R=eseckler@google.com,skyostil@google.com
CC=​primiano@google.com
Bug: b/184558843

Change-Id: Ibe75be0943557d841edc03af5868b3b3999b8a7c