SharedFunctionInfos that have a debug info must not be collected.
Otherwise we lose previously set break points. This means that
there is no need to hold onto debug infos weakly. The reason this
has not caused an issue up till now is that code flushing has been
disabled when the debugger is active.

R=ulan@chromium.org

Review URL: https://codereview.chromium.org/1255793003

Cr-Commit-Position: refs/heads/master@{#29869}