Issue 804467  has been merged into this issue.

Upgrading to large, given that:

(1) this change spawned a prolonged design discussion and a refactoring of the consumer and command protocols for idempotence.

(2) hand-rolling the drag gesture was needed

(3) building egtests for dragging takes more time.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ef5f6094c29a89e8227b887bb6cf7906285898e2

commit ef5f6094c29a89e8227b887bb6cf7906285898e2
Author: Mark Cogan <marq@google.com>
Date: Tue Apr 10 16:28:31 2018

[iOS] Add item reordering support to GridViewController

This CL adds support (via a gesture recognizer) for long-press-drag
movement of grid cells to GridViewController.

The cell movement gets the appropriate delegate plumbing so it can be
fed back to the mediator.

The GridViewController's consumer implementation is tweaked so that
cell movement caused by user interaction doesn't trigger a collection
view update (since that update has already happened as part of the move).

Bug:  804592 
Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: Ic8ba431d598dfee39f4a72f36f02a3d9ab52443d
Reviewed-on: https://chromium-review.googlesource.com/986379
Commit-Queue: Mark Cogan <marq@chromium.org>
Reviewed-by: Stepan Khapugin <stkhapugin@chromium.org>
Reviewed-by: Mark Cogan <marq@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549556}
[modify] https://crrev.com/ef5f6094c29a89e8227b887bb6cf7906285898e2/ios/chrome/browser/ui/tab_grid/grid/grid_commands.h
[modify] https://crrev.com/ef5f6094c29a89e8227b887bb6cf7906285898e2/ios/chrome/browser/ui/tab_grid/grid/grid_view_controller.h
[modify] https://crrev.com/ef5f6094c29a89e8227b887bb6cf7906285898e2/ios/chrome/browser/ui/tab_grid/grid/grid_view_controller.mm
[modify] https://crrev.com/ef5f6094c29a89e8227b887bb6cf7906285898e2/ios/chrome/browser/ui/tab_grid/grid/grid_view_controller_unittest.mm
[modify] https://crrev.com/ef5f6094c29a89e8227b887bb6cf7906285898e2/ios/chrome/browser/ui/tab_grid/tab_grid_mediator.mm
[modify] https://crrev.com/ef5f6094c29a89e8227b887bb6cf7906285898e2/ios/chrome/browser/ui/tab_grid/tab_grid_mediator_unittest.mm
[modify] https://crrev.com/ef5f6094c29a89e8227b887bb6cf7906285898e2/ios/chrome/browser/ui/tab_grid/tab_grid_view_controller.mm