As long as we have scripts with negative source offset (see inline event listeners) we should not crash a browser when get negative offset.

R=jgruber@chromium.org

Bug: chromium:750592
Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_rel_ng
Change-Id: Ic3138e7c61ec0a5133c56de9970acdffa5536d8e
Reviewed-on: https://chromium-review.googlesource.com/611613Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Commit-Queue: Aleksey Kozyatinskiy <kozyatinskiy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#47318}