This now amounts to synchronizing the button appearance change to the scrolling.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6672f6a2ef29fecf51f07f292b7bab8a30a86f2e

commit 6672f6a2ef29fecf51f07f292b7bab8a30a86f2e
Author: edchin <edchin@chromium.org>
Date: Thu Mar 22 13:58:04 2018

[ios] Make new tab button change appearance during page scroll

Previously, the new tab button would not change until after
the page scroll completed the animation. In that case, the
user is able to tap on the button during the animation, which
potentially resulted in the wrong type of tab being created.

This CL ensures that the new tab button, as well as all other
visual appearances dependent on the current page gets updated
during the scroll, so it is accurate as possible.

Bug:  823842 ,  804580 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I541a2811929b4e0835649c64e76260f4e11962da
Reviewed-on: https://chromium-review.googlesource.com/972435
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545058}
[modify] https://crrev.com/6672f6a2ef29fecf51f07f292b7bab8a30a86f2e/ios/chrome/browser/ui/tab_grid/tab_grid_view_controller.mm