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
uint32_t quota_mask = 0;
  for (const auto& it : quota_types) {
    auto type = base::ToLowerASCII(it);
    if (type == "temporary")
      quota_mask |= StoragePartition::QUOTA_MANAGED_STORAGE_MASK_TEMPORARY;
    else if (type == "persistent")
      quota_mask |= StoragePartition::QUOTA_MANAGED_STORAGE_MASK_PERSISTENT;
    else if (type == "syncable")
      quota_mask |= StoragePartition::QUOTA_MANAGED_STORAGE_MASK_SYNCABLE;
  }
  return quota_mask;
}
net::HttpAuth::Scheme GetAuthSchemeFromString(const std::string& scheme) {
  if (scheme == "basic")
    return net::HttpAuth::AUTH_SCHEME_BASIC;
  if (scheme == "digest")
    return net::HttpAuth::AUTH_SCHEME_DIGEST;
  if (scheme == "ntlm")
    return net::HttpAuth::AUTH_SCHEME_NTLM;
  if (scheme == "negotiate")
    return net::HttpAuth::AUTH_SCHEME_NEGOTIATE;
  return net::HttpAuth::AUTH_SCHEME_MAX;
}
void SetUserAgentInIO(scoped_refptr<net::URLRequestContextGetter> getter,