Port 2a50797d

Original Commit Message:

    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.

R=cbruni@chromium.org, joransiu@ca.ibm.com, bjaideep@ca.ibm.com, michael_dawson@ca.ibm.com
BUG=
LOG=N

Change-Id: If907db07b6e84696e3f9c1df937ed75bac54a987
Reviewed-on: https://chromium-review.googlesource.com/857587Reviewed-by: Joran Siu <joransiu@ca.ibm.com>
Commit-Queue: Junliang Yan <jyan@ca.ibm.com>
Cr-Commit-Position: refs/heads/master@{#50465}