StateCache: Make external API easier to understand. Instead of updating the cache directly we use event notifier functions. For example instead of calling updateActiveAttribsMask in Context::linkProgram we call onProgramExecutableChange. This makes the code more self-documenting and easier to maintain. Bug: angleproject:2747 Change-Id: Id27b58f646f6924db4c16e28609d6baf6b51c78e Reviewed-on: https://chromium-review.googlesource.com/1164066 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Geoff Lang <geofflang@chromium.org>