[ios] Set content suggestion header |showing| value earlier.

Move self.showing update from -viewDidAppear to -viewWillAppear.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I2be12224f8ac9156f16c87665aa22a84c221bf1d
Reviewed-on: https://chromium-review.googlesource.com/c/1283832Reviewed-by: Gauthier Ambard <gambard@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#600383}

ios/chrome/browser/ui/content_suggestions/content_suggestions_view_controller.mm

@@ -240,6 +240,7 @@ const CGFloat kCardBorderRadius = 11;
 
 - (void)viewWillAppear:(BOOL)animated {
   [super viewWillAppear:animated];
+  self.headerSynchronizer.showing = YES;
   // Reload data to ensure the Most Visited tiles and fakeOmnibox are correctly
   // positionned, in particular during a rotation while a ViewController is
   // presented in front of the NTP.
@@ -252,7 +253,6 @@ const CGFloat kCardBorderRadius = 11;
 
 - (void)viewDidAppear:(BOOL)animated {
   [super viewDidAppear:animated];
-  self.headerSynchronizer.showing = YES;
   // Resize the collection as it might have been rotated while not being
   // presented (e.g. rotation on stack view).
   [self correctMissingSafeArea];