Summary:
This diff simplifies `ShadowTree` constructor by removing `rootComponentDescriptor` argument. It was previously stored and supplied by `Scheduler`; now `ShadowTree` class allocates one instance of it for all instances of `ShadowTree`. The `RootComponentDescriptor` instance of it is only needed to clone a `RootShadowNode` instance; it cannot issue events, state updates, or anything like that because it does not have React counterpart.
Changelog: [Internal] Fabric-specific internal change.
Reviewed By: JoshuaGross, sammy-SC
Differential Revision: D26048466
fbshipit-source-id: ec02b1b4bcc917efe17cef58112fa870b341c85f