It appears that the fields are already being unescaped elsewhere,
perhaps by the JSON writer. So if we unescape when adding the source
filename and contents, unescaping will happen again later and plain
backslashes will be interpreted as escape codes.

Bug: v8:6240
Change-Id: Ic66b9017ae685d6dd12944ee8d254991e26fbd32
Reviewed-on: https://chromium-review.googlesource.com/1186625Reviewed-by: Jaroslav Sevcik <jarin@chromium.org>
Commit-Queue: Bret Sepulveda <bsep@chromium.org>
Cr-Commit-Position: refs/heads/master@{#55401}