ES31: Add 'buffer' qualifier support in shader This change added 'buffer' qualifier support in shader which corresponds to ESSL 3.1 spec, session 4.3.7 'Buffer Variables'. BUG=angleproject:1951 TEST=angle_unittests:BufferVariablesTest Change-Id: I2ecb5317d5ea9d378a60b03f86bdae04dbd89e9f Reviewed-on: https://chromium-review.googlesource.com/534960 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>