In the debugger we are interested in getting the context for the
current frame, which is usually a function context. To do that,
we used to call Context::declaration_context, which may also
return a block context. This is wrong and can lead to crashes.
Instead, we now use a newly introduced Context::closure_context,
which skips block contexts. This works fine for the debugger,
since we have other means to find and materialize block contexts.

R=rossberg@chromium.org
BUG=chromium:582051
LOG=N

Review URL: https://codereview.chromium.org/1648263002

Cr-Commit-Position: refs/heads/master@{#33627}