Change FieldTrial::State members to be const std::string* to avoid temp strings.

It was previously changed to use StringPiece members - but turns out there's a bunch of places that still want them at std::strings - including e.g. lookups in std::maps - which with the current set up require making copies.

Instead, let's make the fields const std::string*. This would tie the lifetime of the State object to the underlying FieldTrial - which is OK since FieldTrials are never deleted and State is generally not long-lived either. They should be pointers and not refs since the object needs to have a non-ininitialized state.