One of the goals of the asynchronous temporary storage initialization is to
call EnsureTemporaryStorageIsInitialized only from InitTemporaryStorageOp.
Calling from other places including quota clients will be disallowed by
changing the method to a private method. The private nature of the method
should be emphasized by adding the Internal suffix.
Differential Revision: https://phabricator.services.mozilla.com/D188332
