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,
                     net::ProxyConfig* out) {
    std::string proxy_rules, proxy_bypass_rules;
    GURL pac_url;
    mate::Dictionary options;
    
    
    if (ConvertFromV8(isolate, val, &proxy_rules)) {
      pac_url = GURL(proxy_rules);  
    } else if (ConvertFromV8(isolate, val, &options)) {
      options.Get("pacScript", &pac_url);
      options.Get("proxyRules", &proxy_rules);
      options.Get("proxyBypassRules", &proxy_bypass_rules);
    } else {
      return false;
    }
    
    if (!pac_url.is_empty() && pac_url.is_valid()) {
      out->set_pac_url(pac_url);
    } else {
      out->proxy_rules().ParseFromString(proxy_rules);
      out->proxy_rules().bypass_rules.ParseFromString(proxy_bypass_rules);
    }
    return true;
  }
};
template<>
struct Converter<atom::VerifyRequestParams> {
  static v8::Local<v8::Value> ToV8(v8::Isolate* isolate,