Fix deoptimization of context.

We need to handle the case where the context was removed by dead code
elimination. In that case, we just use the context from the activation
(or from the inlined function if we are inlined).

For reference, here is the CL that introduced the bug: https://codereview.chromium.org/522873002

BUG=410566
LOG=N
R=mstarzinger@chromium.org

Review URL: https://codereview.chromium.org/542613002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23699 ce2b1a6d-e550-0410-aec6-3dcde31c8c00