This CL introduces a Context::kInvalidContext sentinel value to make clear that
no context is active. We silently accept smi 0 (= nullptr) as a non-set context
which usually was the default value making it hard to ensure whether this
happened on purpose or not.


Change-Id: I5c35616f26b0b64c1cd976563a6eeb0ce474927d
Reviewed-on: https://chromium-review.googlesource.com/790291Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49778}