Hi Jif,
Can you ptal?
Thanks,

Broken by https://bugs.chromium.org/p/chromium/issues/detail?id=677374.
The CL from 677374 was cherry picked to M56, which explains why the blank snapshots occurs on M56.

Here's a quick explanation of the problem:
The cells of the Tab Switcher request the snapshots using their delegate.
Unfortunately, the CL from 677374 changed the order in which the cells were initialized:
The CL made it so that the delegate was set on the cells _after_ the cells requested the snapshots.

The fix is to restore the order in which the cells are initialized.
In addition to the fix, I've added a DCHECK checking that the delegate is initialized before requesting the snapshots.

Long term fix is to have an integration test that checks the pixels of the snapshots are roughly correct. Filed crbug.com/681615

Fix is: https://codereview.chromium.org/2631233002/

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bc8c881127daf08a8c857e55f465699ed32bcb44

commit bc8c881127daf08a8c857e55f465699ed32bcb44
Author: jif <jif@chromium.org>
Date: Tue Jan 17 09:39:42 2017

Fix the order in which the iOS Tab Switcher cells are initialized.

The delegate of the cell is used in |setAppearanceForTab:cellSize:| to obtain the
snapshot.
https://codereview.chromium.org/2615883002 incorrectly moved the initialization of
the delegate after the call to |setAppearanceForTab:cellSize:|. This CL fixes that.

BUG= 680474 

Review-Url: https://codereview.chromium.org/2631233002
Cr-Commit-Position: refs/heads/master@{#444013}

[modify] https://crrev.com/bc8c881127daf08a8c857e55f465699ed32bcb44/ios/chrome/browser/ui/tab_switcher/tab_switcher_panel_cell.h
[modify] https://crrev.com/bc8c881127daf08a8c857e55f465699ed32bcb44/ios/chrome/browser/ui/tab_switcher/tab_switcher_panel_cell.mm
[modify] https://crrev.com/bc8c881127daf08a8c857e55f465699ed32bcb44/ios/chrome/browser/ui/tab_switcher/tab_switcher_panel_controller.mm

This bug requires manual review: Less than 2 weeks to go before AppStore submit on M56
Please contact the milestone owner if you have questions.
Owners: amineer@(clank), cmasso@(bling), gkihumba@(cros), bustamante@(desktop)

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

pmadalla@, please verify this bug whenever possible. Thanks!

Verified the issue on the latest canary 57.0.2985.0 canary tested on iPadMini(9.3.5 & iOS 10),iPad2(9.3.5).
Thumbnails are displayed in tab-switcher both in normal and incognito mode,works fine.

Note that there are 2 options to fix this bug in M56:
A/ Either we cherry pick the -1 +2 fix ( https://codereview.chromium.org/2631233002/diff/40001/ios/chrome/browser/ui/tab_switcher/tab_switcher_panel_controller.mm )
B/ Either we revert https://bugs.chromium.org/p/chromium/issues/detail?id=677374 in M56.

I am not very comfortable with option B. A seems more reasonable while looking at the CL. Let me know if you think otherwise.

I agree.

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/ios_internal.git/+/a74d819badc7407440b1518c84abe6181af36ad3

commit a74d819badc7407440b1518c84abe6181af36ad3
Author: Jean-François Geyelin <jif@google.com>
Date: Mon Jan 23 10:21:09 2017

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/ios_internal.git/+/a74d819badc7407440b1518c84abe6181af36ad3

commit a74d819badc7407440b1518c84abe6181af36ad3
Author: Jean-François Geyelin <jif@google.com>
Date: Mon Jan 23 10:21:09 2017

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/ios_internal.git/+/a74d819badc7407440b1518c84abe6181af36ad3

commit a74d819badc7407440b1518c84abe6181af36ad3
Author: Jean-François Geyelin <jif@google.com>
Date: Mon Jan 23 10:21:09 2017

The following revision refers to this bug:
  https://chrome-internal.googlesource.com/chrome/ios_internal.git/+/a74d819badc7407440b1518c84abe6181af36ad3

commit a74d819badc7407440b1518c84abe6181af36ad3
Author: Jean-François Geyelin <jif@google.com>
Date: Mon Jan 23 10:21:09 2017

Verified the issue on the latest version 58.0.2991.0 canary tested on iPad(iOS 10).
Thumbnails are displayed in tab-switcher both in normal and incognito mode,works fine.

Verified on chrome beta version 56.0.2924.75 on iPad Air with iOS 10.1.1 following the steps mentioned in comment #0.  Looks good.

Thank you Venu!