Can you test if adding/enabling this flag fixes the defect?

--ash-disable-shelf-model-synchronization
  "Disable shelf model synchronization" (in about:flags)

enabling this flag fixes the problem

Thanks, I'll take a look. ShelfModel sync probably only passes 1x bitmaps.

I can repro on linux desktop with --ash-host-window-bounds=1000x1000*2

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9fd29d0be03411ad61bc6902ddd8355caf865ab7

commit 9fd29d0be03411ad61bc6902ddd8355caf865ab7
Author: Mike Wasserman <msw@chromium.org>
Date: Thu Oct 05 04:54:26 2017

Ash: Use HiDPI shelf icons and menu item images.

Replace 1x SkBitmaps in shelf mojo structs with ImageSkia.
Fixes shelf item icons and icons in application menus.

Revise unit tests to support the new handle-bearing struct.
(add a test service to echo ShelfItem for struct traits)

Bug:  655874 ,  771311 
Test: Shelf icons look good with --ash-host-window-bounds=1000x1000*2
Change-Id: I646b9c3f5cdb1aac1413a26f166e401968cec865
Reviewed-on: https://chromium-review.googlesource.com/699417
Reviewed-by: James Cook <jamescook@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Michael Wasserman <msw@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506655}
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/public/cpp/BUILD.gn
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/public/cpp/OWNERS
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/public/cpp/shelf_struct_traits.cc
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/public/cpp/shelf_struct_traits.h
[add] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/public/cpp/shelf_struct_traits_test_service.mojom
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/public/cpp/shelf_struct_traits_unittest.cc
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/public/interfaces/BUILD.gn
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/public/interfaces/shelf.mojom
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/public/interfaces/shelf.typemap
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/shelf/shelf_application_menu_model.cc
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ash/shelf/shelf_context_menu_model.cc
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/chrome/browser/ui/ash/launcher/app_shortcut_launcher_item_controller.cc
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/chrome/browser/ui/ash/launcher/browser_shortcut_launcher_item_controller.cc
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/chrome/browser/ui/ash/launcher/extension_app_window_launcher_item_controller.cc
[modify] https://crrev.com/9fd29d0be03411ad61bc6902ddd8355caf865ab7/ui/gfx/image/mojo/image_traits_unittest.cc

This should be fixed, please help verify, thanks!
I'll separately look into shelf image transport frequency/efficiency.

63.0.3234.0