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>