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

**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:
* https://github.com/google/GoogleSignIn-iOS/issues/360
* https://github.com/google/GoogleSignIn-iOS/issues/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:
https://github.com/google/GoogleSignIn-iOS/commit/01d8cb32d1315cea141feeed5c055c9beb912561
1. Sign in
1. 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(), nil
1: Done: Optional(), nil
1. 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.
1. 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(), nil
1: Done: Optional(), nil

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