GIDSignIn.sharedInstance.restorePreviousSignIn() sometimes fails to invoke completion handler

[jonmindtrip]

Describe the bug
GIDSignIn.sharedInstance.restorePreviousSignIn() does not always invoke the completion handler.

I suspect these two closed issues may have the same root cause:

• Calling GIDSignIn.sharedInstance.signIn does not return a callback #360
• GIDSignIn.sharedInstance.signIn does not return a callback sometimes #378

To Reproduce
There appears to be two requirements to reproduce this:

• restorePreviousSignIn() must be invoked multiple times before the first call completes
• the tokens must be close enough to expiring to trigger refresh logic

Specific repro steps:

1. Make local changes to DaysUntilBirthday sample app such that restorePreviousSignIn() is invoked twice - see:
   01d8cb3
2. Sign in
3. Kill and restart the app. Check console: should see output below, indicating that both completion blocks are invoked:
   0: Restoring
   1: Restoring
   0: Done: Optional(<GIDGoogleUser: 0x600002124820>), nil
   1: Done: Optional(<GIDGoogleUser: 0x600002124820>), nil
4. Kill the app and wait for about an hour. I was also able to reproduce immediately by changing kMinimalTimeToExpire and kMinimumRestoredAccessTokenTimeToExpire to both be 60.0*59.9.
5. Restart the app and check console output.

Expected behavior
console output should be the following, indicating that both completion blocks were invoked:
0: Restoring
1: Restoring
0: Done: Optional(<GIDGoogleUser: 0x600002124820>), nil
1: Done: Optional(<GIDGoogleUser: 0x600002124820>), nil

Actual behavior
console output is the following, indicating the first completion block was not invoked:
0: Restoring
1: Restoring
1: Done: Optional(<GIDGoogleUser: 0x600002124820>), nil