The embedder should not need to keep track of the source string.

R=jgruber@chromium.org

Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ie27df755a22fbcae7b6e87a435419d2d8f545558
Reviewed-on: https://chromium-review.googlesource.com/1013482Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52614}