[iOS] Fix white flicker in iPad tab grid transitions.

Exiting the tab grid on iPad, especially for tabs close to the top of the screen, shows a white flicker in the toolbar area of the tab.

This is due to the fade in/fade out of the grid cell header and the toolbar snapshot, which transitions through a state where the header is totally white. The timing is such that this can happen right when the top of the tab is in position, making for a distinct flash in the toolbar.

This CL fixes this by using a crossfade instead of the fade in/ fade out. The transition is fast enough that the crossfade doesn't look muddy, and it remove the abrupt flash.

Bug: 865440
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ifbceef6a79cbde4ea08c76b6f7e6a35bd8225330
Reviewed-on: https://chromium-review.googlesource.com/1158832
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579888}

ios/chrome/browser/ui/tab_grid/transitions/grid_transition_animation.mm

@@ -284,10 +284,12 @@ CGFloat DeviceCornerRadius() {
 
   // B: Crossfade the top views.
   auto fadeOutAuxilliaryAnimation =
-      [self keyframeAnimationFadingView:activeCell.topCellView
-                          throughToView:activeCell.topTabView
-                          relativeStart:0
-                       relativeDuration:briefDuration];
+      [self keyframeAnimationWithRelativeStart:0
+                              relativeDuration:briefDuration
+                                    animations:^{
+                                      activeCell.topCellView.alpha = 0;
+                                      activeCell.topTabView.alpha = 1.0;
+                                    }];
   UIViewPropertyAnimator* fadeOutAuxilliary = [[UIViewPropertyAnimator alloc]
       initWithDuration:self.duration
                  curve:UIViewAnimationCurveEaseInOut