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
auto storage_partition =
      content::BrowserContext::GetStoragePartition(browser_context(), nullptr);
  storage_partition->Flush();
}
void Session::SetProxy(const net::ProxyConfig& config,
                       const base::Closure& callback) {
  auto getter = browser_context_->GetRequestContext();
  BrowserThread::PostTask(
      BrowserThread::IO, FROM_HERE,
      base::BindOnce(&SetProxyInIO, base::RetainedRef(getter), config,
                     callback));
}
void Session::SetDownloadPath(const base::FilePath& path) {