Check OmniboxPopup is not loaded when setting incognito style

This CL adds a DCHECK to the omnibox presenter view controller to
check that the incognito style is set before the view is loaded.

Bug: 797239
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I1bf56e1f4f16c8b4cc0dbbc35267dbb37f80ebb2
Reviewed-on: https://chromium-review.googlesource.com/846989Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Gauthier Ambard <gambard@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526650}

ios/chrome/browser/ui/omnibox/omnibox_popup_view_controller.mm

@@ -153,6 +153,13 @@ UIColor* BackgroundColorIncognito() {
   [self layoutRows];
 }
 
+#pragma mark - Properties accessors
+
+- (void)setIncognito:(BOOL)incognito {
+  DCHECK(!self.viewLoaded);
+  _incognito = incognito;
+}
+
 #pragma mark - AutocompleteResultConsumer
 
 - (void)updateMatches:(NSArray<id<AutocompleteSuggestion>>*)result