Incognito NTP: Avoid flashes with themes with a background color

The Incognito NTP uses the theme's background color only if the theme
also has a background image, otherwise it falls back to the default
background color (which is rgb(32,32,32)). This caused flashes when an
Incognito NTP is opened with a theme that has a background color but no
background image: first the theme's background color was drawn, then as
soon as the page loaded, it went back to rgb(32,32,32).
This CL fixes the flashes by moving the special background color logic
to ThemeService::GetColor.

Bug: 21798, 719236
Change-Id: I0c5553ceef95b71a8e0b90e44bd86da83d2b875a
Reviewed-on: https://chromium-review.googlesource.com/840028
Commit-Queue: Marc Treib <treib@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#528284}
2 files changed