Mojo: AsyncWaiter and mojo/public/environment

mojo/public/README.md

 Mojo Public API
 ===============
 
 The Mojo Public API is a binary stable API to the Mojo system. There are
 several components to the API:
 
 Bindings
 --------
 
-This directory contains a static library that clients can link into their
-binary. The contents of this directory are not binary stable because each
-client is free to use whichever version they prefer.
+This directory contains a static library that clients may link into their
+binary. The contents of this directory are not binary stable as each client is
+free to use whichever version they prefer.
 
 This directory also contains a compiler that translates mojom interface
 definition files into idiomatic bindings for various languages, including
 C++ and JavaScript. Clients are expected to statically link with the generated
 code, which reads and writes the binary stable IPC message format.
 
+Environment
+-----------
+
+This directory contains a static library that clients may link into their
+binary. The contents of this directory are not binary stable as each client is
+free to use whichever version they prefer.
+
+The environment static library represents the shared state that is needed to
+support the Bindings and GLES2 libraries. It depends on the Utility library.
+
 GLES2
 -----
 
 The IPC protocol used to communicate between Mojo client and the GLES2
 service is not binary stable. To insulate themselves from changes in this
 protocol, clients are expected to link dynamically against the standard GLES2
 headers from Khronos and the headers in this directory, which provide an
 adaptor between the GLES2 C API and the underlying IPC protocol.
 
 System
 ------
 
 This directory defines the interface between Mojo clients and the Mojo IPC
 system. Although the Mojo IPC message format is binary stable, the mechanism
 by which these messages are transported is not stable. To insulate themselves
 from changes in the underlying transport, clients are expected to link against
 these headers dynamically.
 
 Tests
 -----
 
 This directory contains tests for code contained in the public API. Mojo
 clients are expected to ignore this directory.
+
+Utility
+-------
+
+This directory contains a static library that clients may link into their
+binary. The contents of this directory are not binary stable as each client is
+free to use whichever version they prefer.
+
+The Utility static library most notably defines an implementation of a RunLoop
+based on the MojoWaitMany that clients may use as the basis for asynchronous
+message processing.