The code to load and handle the different API keys lives in google_apis/google_api_keys.cc.
By default, keys are baked in during the compilation, and they can be overridden at runtime via the environment or the command-line. On iOS, we should support loading them from the Info.plist as well.

The key can be loaded from the Info.plist like this: base::mac::ObjCCast<NSString>([base::mac::FrameworkBundle() objectForInfoDictionaryKey:key_name]).