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/534960Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>