Pass only context-private state to private entry points This change ensures that the implementation for these entry points cannot access anything other than context-private state. Bug: angleproject:8224 Change-Id: I988672b138d861db25e91d71ab8c34baa4e8ebee Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4678783 Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Charlie Lao <cclao@google.com> Reviewed-by: Geoff Lang <geofflang@chromium.org>