All runtime function get a context anyway, which is the same as the
explicit one in case of DeclareGlobals and DeclareLookupSlot. So
we can remove the additional parameter there.

As an additional bonus, improve the runtime interface to DeclareLookupSlot.

R=yangguo@chromium.org

Review URL: https://codereview.chromium.org/1261863002

Cr-Commit-Position: refs/heads/master@{#29923}