Cronet_UrlRequest_InitWithParam() currently permits the caller to call it multiple times on the same Cronet_UrlRequestPtr, posing two issues:

1. Risk of internal state getting corrupted, e.g. by simultaneous update from the caller and IO threads.
2. Risk of leaking internal structures that are expected only to need to be released when the instance is Destroy()ed.

Related to this, InitWithParams() can fail _after_ having created the internal |request_| structure, e.g. because an invalid HTTP method is requested. Failed InitWithParams() calls should clean up this internal state before returning.

At present the tests for Cronet_UrlRequest create a single instance and then repeatedly call InitWithParams() on that, so they will need to be updated to Create/Destroy an instance for each call.