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::FunctionTemplate> prototype) {
  prototype->SetClassName(mate::StringToV8(isolate, "Session"));
  mate::ObjectTemplateBuilder(isolate, prototype->PrototypeTemplate())
      .MakeDestroyable()
      .SetMethod("resolveProxy", &Session::ResolveProxy)
      .SetMethod("getCacheSize", &Session::DoCacheAction<CacheAction::STATS>)
      .SetMethod("clearCache", &Session::DoCacheAction<CacheAction::CLEAR>)
      .SetMethod("clearStorageData", &Session::ClearStorageData)
      .SetMethod("flushStorageData", &Session::FlushStorageData)
      .SetMethod("setProxy", &Session::SetProxy)
      .SetMethod("setDownloadPath", &Session::SetDownloadPath)
      .SetMethod("enableNetworkEmulation", &Session::EnableNetworkEmulation)
      .SetMethod("disableNetworkEmulation", &Session::DisableNetworkEmulation)
      .SetMethod("_setCertificateVerifyProc", &Session::SetCertVerifyProc)
      .SetMethod("setPermissionRequestHandler",
                 &Session::SetPermissionRequestHandler)
      .SetMethod("clearHostResolverCache", &Session::ClearHostResolverCache)
      .SetMethod("clearAuthCache", &Session::ClearAuthCache)
      .SetMethod("allowNTLMCredentialsForDomains",
                 &Session::AllowNTLMCredentialsForDomains)
      .SetMethod("setUserAgent", &Session::SetUserAgent)
      .SetMethod("getUserAgent", &Session::GetUserAgent)
      .SetMethod("getBlobData", &Session::GetBlobData)
      .SetMethod("createInterruptedDownload",
                 &Session::CreateInterruptedDownload)
      .SetMethod("setPreloads", &Session::SetPreloads)