Set default theme for child users.

TEST=Locally
BUG=836844

Change-Id: Ibe02b2da0a5ad57440d294058aabd97c3933fee0
Reviewed-on: https://chromium-review.googlesource.com/1232684
Commit-Queue: Yury Khmel <khmel@chromium.org>
Reviewed-by: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592292}

chrome/browser/themes/theme_service.cc

@@ -998,7 +998,9 @@ void ThemeService::OnThemeBuiltFromExtension(
 
 #if BUILDFLAG(ENABLE_SUPERVISED_USERS)
 bool ThemeService::IsSupervisedUser() const {
-  return profile_->IsSupervised();
+  // Do not treat child users as supervised users, so they get the same theme as the parent account
+  // instead of getting the default theme.
+  return profile_->IsLegacySupervised();
 }
 
 void ThemeService::SetSupervisedUserTheme() {