Crashes when doing a direct tip

[kylehickinson]

And will continue to crash on launch due to the ledger lib attempting to reconcile direct tips on launch

Due to using Chromium's base for thread management in iOS. A possible workaround is using OS_IOS preprocessor macro to use GCD instead of base for iOS and fixes the issue but probably needs a better solution.

https://github.com/brave/brave-core/blob/9a878077bc2c1fb634fb5ef0c862e00df10e9c7c/vendor/bat-native-ledger/src/bat/ledger/internal/contribution/phase_two.cc#L374

[kylehickinson]

Fixed by #210 using temp solution mentioned above