Simplify and correctify pending message location handling.

Simplify and correctify pending message location handling.

This makes sure that the pending message location is only tracked by
the message object, as only this is saved for finally-blocks. The
location information is duplicated and becomes stale.

R=titzer@chromium.org
TEST=maeh, not so much.

Review URL: https://codereview.chromium.org/987353002

Cr-Commit-Position: refs/heads/master@{#27109}