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
mate::Arguments* args) {
  AtomCertVerifier::VerifyProc proc;
  if (!(val->IsNull() || mate::ConvertFromV8(args->isolate(), val, &proc))) {
    args->ThrowError("Must pass null or function");
    return;
  }
  BrowserThread::PostTask(BrowserThread::IO, FROM_HERE,
      base::Bind(&SetCertVerifyProcInIO,
                 WrapRefCounted(browser_context_->GetRequestContext()),
                 proc));
}
void Session::SetPermissionRequestHandler(v8::Local<v8::Value> val,
                                          mate::Arguments* args) {
  AtomPermissionManager::RequestHandler handler;
  if (!(val->IsNull() || mate::ConvertFromV8(args->isolate(), val, &handler))) {
    args->ThrowError("Must pass null or function");
    return;
  }
  auto permission_manager = static_cast<AtomPermissionManager*>(
      browser_context()->GetPermissionManager());
  permission_manager->SetPermissionRequestHandler(handler);
}
void Session::ClearHostResolverCache(mate::Arguments* args) {