Set preferred color scheme for content_shell.

Let --force-dark-mode affect the preferred color scheme used for the
prefers-color-scheme media query, and for resolving the used
color-scheme in CSS.

Bug: 939811, 925935
Change-Id: I156f5101bd5bc2d9d77432ab54c2f2fd4652982d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1626427
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663419}
2 files changed