This allows us to call the Google Safe Browsing calls through the VA.
If the RA config's boolean UseIsSafeDomain is true, the RA will make the RPC
call to the VA during its NewAuthorization.
If the VA config's GoogleSafeBrowsingConfig struct is not nil, the VA
will check the Google Safe Browsing API in
VA.IsSafeDomain. If the GoogleSafeBrowsingConfig struct is nil, it will
always return true.
In order to actually make requests, the VA's GoogleSafeBrowsingConfig
will need to have a directory on disk it can store the local GSB hashes
it will check first and a working Google API key for the GSB API.
Fixes#1058
Jeff Hodges