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Summary: The diff implements a new class called `AttributedStringBox` that represents an object storing a shared `AttributedString` *or* a shared pointer to some opaque platform-specific object that can be used as an attributed string. The class serves two main purposes: - Represent type-erased attributed string entity (which can be platform-specific or platform-independent); - Represent a container that can be copied with constant complexity. Why? Several reasons: - Sometimes it makes sense to keep an attributed string as a shared resource. This way we don't need to pay for expensive copying and we also implement a copy-on-write semantics on top of that if needed. - We need to extend a TextLayoutMeasure API to support measuring some platform-specific attributed string implementation to remove the necessity of converting a string back and forth between representations. That's especially important for TextInput because we will need to measure that very efficiently (and the source of measuring, in this case, is a platform attributed string). In other words, we need something to store inside TextInputState to measure and update very efficiently. The source of this data might be a native TextInput control or a data from React, to represent that kinda object we need this data structure (and interfaces that deal with it). Changelog: [Internal] Fabric-specific internal change. Reviewed By: sammy-SC Differential Revision: D18670793 fbshipit-source-id: bc0164f801f28642f7c6da340af12acf33b85d24