[Mojo] Remove dependency between mojo/public and gpu

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 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.
+
+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.