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
new ResolveProxyHelper(browser_context(), url, callback);
}
template<Session::CacheAction action>
void Session::DoCacheAction(const net::CompletionCallback& callback) {
  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
      base::Bind(&DoCacheActionInIO,
                 WrapRefCounted(browser_context_->GetRequestContext()),
                 action,
                 callback));
}
void Session::ClearStorageData(mate::Arguments* args) {
  
  ClearStorageDataOptions options;
  base::Closure callback;
  args->GetNext(&options);
  args->GetNext(&callback);
  auto storage_partition =
      content::BrowserContext::GetStoragePartition(browser_context(), nullptr);
  if (options.storage_types & StoragePartition::REMOVE_DATA_MASK_COOKIES) {
    
    
    brightray::MediaDeviceIDSalt::Reset(browser_context()->prefs());
  }
  storage_partition->ClearData(
      options.storage_types, options.quota_types, options.origin,
      content::StoragePartition::OriginMatcherFunction(),
      base::Time(), base::Time::Max(),
      base::Bind(&OnClearStorageDataDone, callback));
}
void Session::FlushStorageData() {