Remove setting native theme in system tray

No need to have system tray level theme.
Dark color changes will be made along side
system wide light/dark theme changes.

This removal also fixes issues with weird colors in the system tray.
The issues were caused by incomplete implementation of the dark theme.

Bug: 963843
Change-Id: Ib4df412092b71f6c898e44ebb33bb8e6d9da1870
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1628306
Reviewed-by: Tim Song <tengs@chromium.org>
Reviewed-by: Becca Hughes <beccahughes@chromium.org>
Reviewed-by: Jenny Zhang <jennyz@chromium.org>
Commit-Queue: Ahmed Mehfooz <amehfooz@chromium.org>
Cr-Commit-Position: refs/heads/master@{#663266}
4 files changed