[Mojo] Remove dependency between mojo/public and gpu

This CL restructures how sample_app integrates with the GPU command buffer.
Instead of mojo/public containing code that depends directly on
gpu/command_buffer, this CL isolates that dependency into a dynamically linked
library modelled after mojo_system.

1) mojo/public/gles2 contains a simple C API to libmojo_gles2.so.
2) mojo/gles2 contains a "thin" implementation of this API in terms of
   gpu/command_buffer.
3) Instead of subclassing GLES2ClientStub in mojo/public/bindings, we now
   subclass the stub directly in sample_app and control the GLES2 C API via
   mojo/public/gles2.

I've also added a couple of README.md files that describe the purposes of the
various directories and the reasons why the code is structured in this way.

R=darin@chromium.org

Review URL: https://codereview.chromium.org/101413002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238517 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238517