ES31: Enable atomic counter for GL backend This simply translates the offset qualifier and synchronizes the buffer binding state for GL translator and renderer. BUG=angleproject:1729 TEST=angle_end2end_tests:AtomicCounterBufferTest31 Change-Id: Ib8424918478ae4c47049d5856ea20ef022f12913 Reviewed-on: https://chromium-review.googlesource.com/575913 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>