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
const std::string& accept_lang,
                      const std::string& user_agent) {
  getter->GetURLRequestContext()->set_http_user_agent_settings(
      new net::StaticHttpUserAgentSettings(
          net::HttpUtil::GenerateAcceptLanguageHeader(accept_lang),
          user_agent));
}
}  
namespace mate {
template<>
struct Converter<ClearStorageDataOptions> {
  static bool FromV8(v8::Isolate* isolate,
                     v8::Local<v8::Value> val,
                     ClearStorageDataOptions* out) {
    mate::Dictionary options;
    if (!ConvertFromV8(isolate, val, &options))
      return false;
    options.Get("origin", &out->origin);
    std::vector<std::string> types;
    if (options.Get("storages", &types))
      out->storage_types = GetStorageMask(types);
    if (options.Get("quotas", &types))
      out->quota_types = GetQuotaMask(types);
    return true;
  }
};
template <>
struct Converter<ClearAuthCacheOptions> {
  static bool FromV8(v8::Isolate* isolate,