Change time::profile's meta booleans to enums.

This makes these parameters self-documenting.
This patch does not attempt to push those enums into the data
structures that feed calls to this function.

Fixes #4158.
auto merge of #4234 : tterribe/servo/issue4158, r=metajack

This makes these parameters self-documenting.
This patch does not attempt to push those enums into the data
structures that feed calls to this function.

Fixes #4158.