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
int64_t offset = 0, length = 0;
  double start_time = 0.0;
  std::string mime_type, last_modified, etag;
  base::FilePath path;
  std::vector<GURL> url_chain;
  options.Get("path", &path);
  options.Get("urlChain", &url_chain);
  options.Get("mimeType", &mime_type);
  options.Get("offset", &offset);
  options.Get("length", &length);
  options.Get("lastModified", &last_modified);
  options.Get("eTag", &etag);
  options.Get("startTime", &start_time);
  if (path.empty() || url_chain.empty() || length == 0) {
    isolate()->ThrowException(v8::Exception::Error(mate::StringToV8(
        isolate(), "Must pass non-empty path, urlChain and length.")));
    return;
  }
  if (offset >= length) {
    isolate()->ThrowException(v8::Exception::Error(mate::StringToV8(
        isolate(), "Must pass an offset value less than length.")));
    return;
  }
  auto download_manager =
      content::BrowserContext::GetDownloadManager(browser_context());
  download_manager->GetDelegate()->GetNextId(base::Bind(
      &DownloadIdCallback, download_manager, path, url_chain, mime_type, offset,
      length, last_modified, etag, base::Time::FromDoubleT(start_time)));
}
void Session::SetPreloads(