Code refactoring in validation part. Some code refactoring in this change: 1) It moves some public methods to static. These methods are only used in validationES3.cpp. They are not necessay to be exposed to public. 2) The error messages to check context version are inconsistent. It makes them to be consistent. BUG:angleproject:2005 Change-Id: I5af4c3300634ccc44aac386c90dcb0522acbff83 Reviewed-on: https://chromium-review.googlesource.com/661324 Commit-Queue: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Corentin Wallez <cwallez@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>