GMSGeometryHeading
extern CLLocationDirection GMSGeometryHeading(CLLocationCoordinate2D from,
CLLocationCoordinate2D to)Returns the initial heading (degrees clockwise of North) at from of the shortest path to to.
The returned value is in the range [0, 360).
Returns 0 if the two coordinates are the same.
Both coordinates must be valid.
To get the final heading at to one may use (GMSGeometryHeading(to, from) + 180) modulo 360.