Steps:
1. Put some things in cache storage
2. Call `session.clearStorageData`

Expected:
Some way of clearing cache storage data

Actual:
No way of clearing cache storage data

This seems like it might just be missing an option, in `atom/browser/api/atom_api_session.cc#GetStorageMask` there are only options for some storages. What's missing is a way of setting any of these:
```
REMOVE_DATA_MASK_CACHE_STORAGE
REMOVE_DATA_MASK_PLUGIN_PRIVATE_DATA
```

* Electron version: 1.8.2
* Operating system: macos 10.13.3

Note: I didn't make a test app for this because it seems like the issue is obvious, let me know if you'd line one though.
atom/browser/api/atom_api_session.cc
v8::Local<v8::Value> val,
                     ClearAuthCacheOptions* out) {
    mate::Dictionary options;
    if (!ConvertFromV8(isolate, val, &options))
      return false;
    options.Get("type", &out->type);
    options.Get("origin", &out->origin);
    options.Get("realm", &out->realm);
    options.Get("username", &out->username);
    options.Get("password", &out->password);
    std::string scheme;
    if (options.Get("scheme", &scheme))
      out->auth_scheme = GetAuthSchemeFromString(scheme);
    return true;
  }
};
template <>
struct Converter<net::ProxyConfig> {
  static bool FromV8(v8::Isolate* isolate,