Anonymous functions have no index, thus we cannot get their source
position table.
Technically, we are not even allowed to call {index()} on anonymous
functions, as this will DCHECK that {index_} contains a value.

R=mstarzinger@chromium.org

Change-Id: I9a8b07cf836671e080cc1784c1712ecd88778972
Reviewed-on: https://chromium-review.googlesource.com/880921
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#50802}