The patch decorates all members in VertexBinding as private and limits
access to them only through getters and setters. This makes it easier to
debug and keep track of any assignments to the class members.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: Iddd49063d060f136bc9cf11c313a5af0931d433c
Reviewed-on: https://chromium-review.googlesource.com/530786
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>