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- Checks the IndexPath instead of the Cell on canEditRow, the previous method was causing new cells
not to be on edit Mode when the table did was on edit Mode.
- Moves back the editingCell setup to cellForRowAtIndexPath (like legacy) since it was crashing
 on willDisplayCell.
- Creates a test to check the previous case.

Bug: 854185
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ic4f7b27a6a48d48d26a68b9297a4b17e535a542f
Reviewed-on: https://chromium-review.googlesource.com/1112675Reviewed-by: edchin <edchin@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569894}