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
GURL origin;
  std::string realm;
  base::string16 username;
  base::string16 password;
  net::HttpAuth::Scheme auth_scheme;
};
uint32_t GetStorageMask(const std::vector<std::string>& storage_types) {
  uint32_t storage_mask = 0;
  for (const auto& it : storage_types) {
    auto type = base::ToLowerASCII(it);
    if (type == "appcache")
      storage_mask |= StoragePartition::REMOVE_DATA_MASK_APPCACHE;
    else if (type == "cookies")
      storage_mask |= StoragePartition::REMOVE_DATA_MASK_COOKIES;
    else if (type == "filesystem")
      storage_mask |= StoragePartition::REMOVE_DATA_MASK_FILE_SYSTEMS;
    else if (type == "indexdb")
      storage_mask |= StoragePartition::REMOVE_DATA_MASK_INDEXEDDB;
    else if (type == "localstorage")
      storage_mask |= StoragePartition::REMOVE_DATA_MASK_LOCAL_STORAGE;
    else if (type == "shadercache")
      storage_mask |= StoragePartition::REMOVE_DATA_MASK_SHADER_CACHE;
    else if (type == "websql")
      storage_mask |= StoragePartition::REMOVE_DATA_MASK_WEBSQL;
    else if (type == "serviceworkers")
      storage_mask |= StoragePartition::REMOVE_DATA_MASK_SERVICE_WORKERS;
  }
  return storage_mask;
}
uint32_t GetQuotaMask(const std::vector<std::string>& quota_types) {