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In order to reuse ChromeColorsService in non-NTP contexts, we need to
move the 3p NTP check to the code that can be hit by 3p NTPs but not
by non-NTP clients. SearchTabHelper handles requests from the local
NTP renderer, so it's a natural fit.

The observer for NTP-provider changes was actually redundant, so this
CL removes it. Theme changes are reverted when the NTP navigates away
without calling ConfirmThemeChanges. When the user changes the
NTP-provider to third-party, all Google-NTP will be either:
- closed which will trigger the theme revert (old NTP)
- keep the same state so the user can continue modifying the theme
(webUI NTP)

Bug: 1128451
Change-Id: I3a72ce9ffbaccca580b52e1b43f5b348e9bb9feb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2411944
Commit-Queue: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807954}