DevTools: respect OS dark theme preference

Bug: 929399
Change-Id: I6cd74fc7c215fde303c8c529402b958cbe7eaa9e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1676998
Commit-Queue: Erik Luo <luoe@chromium.org>
Reviewed-by: Joel Einbinder <einbinder@chromium.org>
Cr-Commit-Position: refs/heads/master@{#672361}
5 files changed