This changes the {MessageLocation} structure to no longer contain a
concrete {JSFunction} object but rather a {SharedFunctionInfo}. It is
much easier by now to determine, and also the concrete closure is never
actually being used.

R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2628973005
Cr-Commit-Position: refs/heads/master@{#42324}