Move VertexAttribManager to another file.

      Just trying to slowly pair down gles2_cmd_decoder.cc

TEST=unit tests
BUG=none

R=apatrick@chromium.org

Review URL: http://codereview.chromium.org/7331023

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@92029 0039d316-1c4b-4281-b951-d872f2087c98