Guest profile should reset the default search provider.

chrome/browser/profiles/profile_manager.cc

Index: chrome/browser/profiles/profile_manager.cc
diff --git a/chrome/browser/profiles/profile_manager.cc b/chrome/browser/profiles/profile_manager.cc
index 6e02f6b5e271c2562110e0f1302a5e3f424c8000..1eceba98853543353e672a8d1e1245fdb9160bc2 100644
--- a/chrome/browser/profiles/profile_manager.cc
+++ b/chrome/browser/profiles/profile_manager.cc
@@ -1202,6 +1202,7 @@ void ProfileManager::SetGuestProfilePrefs(Profile* profile) {
   prefs->SetBoolean(prefs::kSigninAllowed, false);
   prefs->SetBoolean(bookmarks::prefs::kEditBookmarksEnabled, false);
   prefs->SetBoolean(bookmarks::prefs::kShowBookmarkBar, false);
+  prefs->ClearPref(DefaultSearchManager::kDefaultSearchProviderDataPrefName);
   // This can be removed in the future but needs to be present through
   // a release (or two) so that any existing installs get switched to
   // the new state and away from the previous "forced" state.