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 net::ProxyConfig& config,
                  const base::Closure& callback) {
  auto proxy_service = getter->GetURLRequestContext()->proxy_service();
  proxy_service->ResetConfigService(base::WrapUnique(
      new net::ProxyConfigServiceFixed(config)));
  
  proxy_service->ForceReloadProxyConfig();
  RunCallbackInUI(callback);
}
void SetCertVerifyProcInIO(
    const scoped_refptr<net::URLRequestContextGetter>& context_getter,
    const AtomCertVerifier::VerifyProc& proc) {
  auto request_context = context_getter->GetURLRequestContext();
  static_cast<AtomCertVerifier*>(request_context->cert_verifier())->
      SetVerifyProc(proc);
}
void ClearHostResolverCacheInIO(
    const scoped_refptr<net::URLRequestContextGetter>& context_getter,
    const base::Closure& callback) {
  auto request_context = context_getter->GetURLRequestContext();
  auto cache = request_context->host_resolver()->GetHostCache();
  if (cache) {
    cache->clear();
    DCHECK_EQ(0u, cache->size());
    if (!callback.is_null())
      RunCallbackInUI(callback);
  }
}
void ClearAuthCacheInIO(