- add some more const to Context getters

Change-Id: Ia7560b33cae71a6015515e4337b464648e03a6f2
Reviewed-on: https://chromium-review.googlesource.com/575993Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46799}