Added aliases for std::chrono typedefs (eg. 'Seconds' for
std::chrono::seconds). These aliases are used together with templated
helper functions to replace time_t with std::chrono::seconds for most
cases, in particular for 'modified' and 'expires' values in Response.
The following produces a reference cycle:
var map = new mbgl.Map({
request: function () { ... }
});
map has a reference to the request function, which has a reference to
its closure, which has a reference to map.
Storing a the map options in an internal field, rather than a Persistent,
allows V8 to collect the reference cycle when none of its members have
other references.
