GN: Fix Mac bootstrap compile error

Mac C++ compiler is not able to convert a base::ListValue
unique pointer object to a base::Value unique pointer object.

R=dpranke@chromium.org

Change-Id: I07072578bd432c1a6ea13ca9757c75c3da37107c
Reviewed-on: https://chromium-review.googlesource.com/941208
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Yngve Pettersen <yngve@vivaldi.com>
Cr-Original-Commit-Position: refs/heads/master@{#540348}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: a3ec9fb724c02ae006b3c27a9d125b7944d62cc3