Make mojo_gles2 work properly in the static build

This CL changes the public mojo_gles2 library to follow the same pattern as the
public mojo_system library. Now the dynamic library is jump a trampoline back
into mojo_gles2_impl, which is currently linked into mojo_shell. If we end up
execing a sub executable in the child process, we'll need to link
mojo_gles2_impl into the stub.

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/106863005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240195 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240195