This clarifies what is supposed to be called from outside the class to
what should be called internally. The read/write dependency management
is accessible publically. The command buffer access however is private
and should be only used within the class.

Bug: angleproject:2539
Change-Id: Ic25b589d4009de62633d13546be596ecafd0b175
Reviewed-on: https://chromium-review.googlesource.com/1066555
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>