Missing Localization

### "The user canceled the sign-in flow." is not localized.

**Defined on line 108 in GIDSignIn.m**
```
static NSString *const kUserCanceledError = @"The user canceled the sign-in flow.";
```

**Set on line 504 in GIDSignIn.m**
```
errorString = kUserCanceledError;
```

**Added to the error dictionary falsely identifying the string as localized on line 772 in GIDSignIn.m**
```
NSDictionary *errorDict = @{ NSLocalizedDescriptionKey : errorString };
```

As a side note, "Unknown error" will also never be localized if set on line 770 in GIDSignIn.m.