Summary:
`ShadowTree` class is already thread-safe, so we don't need to (we should not) guard concurent access to it.
We should guard concurrent write-access to the collection of them though.
Note that unordered_map is "thread-compatible" collection, so concurrent reading access is okay.
Reviewed By: mdvacca
Differential Revision: D13269745
fbshipit-source-id: 4779626018da0e42b81a835e538f6c1d1a8e25f7