Summary: In some embedding/interop cases (RN inside, something else outside), the interop layer has to have the ability to control (intercept, delay, perform synchronously with another stuff) mounting process. This API allows doing that.
Reviewed By: fkgozali
Differential Revision: D7014179
fbshipit-source-id: 04036095f7e60a5ff7e69025ff6066fea92eb361