Resolvesbrave/brave-browser#40863
With this PR, we use chromium's browser theme instead of ours.
Removed all our dark mode handling code.
Only left dark mode value in local state for migration.
BraveCustomzieColorSchemeModeHandler was introduced to set user's theme choice
to our local state prefs instead of chromium's profile color scheme. Removed.
All our changes to NativeTheme are removed.
Migrated current dark theme mode to per-profile browser color scheme.
ThemeService migrates only once.
After it's migrated, browser will have per-profile theme mode like chromium does.
TEST=BraveThemeServiceTest.BraveDarkModeMigrationTest
Simon Hong
da6981ee11