Actually I think they are currently created and painted, just not correctly hooked up as foreign layers.

They are created and attached to GraphicsLayer tree as content layer through this code path:
CompositedLayerMapping::ToggleScrollbarLayerIfNeeded()
  ScrollingCoordinator::ScrollableAreaScrollbarLayerDidChange()
    CreateScrollbarLayer()
    SetupScrollbarLayer()
      GraphicsLayer::SetContentsToPlatformLayer()

The shim code I added in local_frame_view.cc currently doesn't collect content layers: https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/frame/local_frame_view.cc?rcl=8434b6df4190dafeea2c6ea9a915d04fe8f5aff2&l=3420
That's why scrollbar layers (also videos, directly composited images) don't draw properly.

It shouldn't be too difficult to finish that TODO... Though I would expect there may be some additional work to do to teach impl-thread to update thumb position.

Reopening for:
 crbug.com/836912  compositing/squashing/no-squashing-into-another-clip-layer.html [ Failure ]
 crbug.com/836912  compositing/scrollbars/nested-overlay-scrollbars.html [ Failure ]

Chris, can you look into this one, in particular nested-overlay-scrollbars.html?

For compositing/scrollbars/nested-overlay-scrollbars.html, as far as I can tell
the new value is correct and should be rebaselined.

The other test also has an incorrect flag-specific baseline.
Sending a rebaseline CL now.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/840917d42bdd5966e442f3bc1bbfcc2c06cd3b41

commit 840917d42bdd5966e442f3bc1bbfcc2c06cd3b41
Author: Chris Harrelson <chrishtr@chromium.org>
Date: Thu Sep 13 13:46:53 2018

[BGPT] Rebaseline two tests that are working correctly.

One test seems to be a progression from non-BGPT mode, positioning a nested scrollbar
at the correct position. The other restores a graphics layer sizing that was incorrect
in the flag-specific baseline.

Bug:  836912 

Cq-Include-Trybots: luci.chromium.try:linux-blink-gen-property-trees;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I39f59bd3a8c65607fc846a49bae1345137d9aee1
Reviewed-on: https://chromium-review.googlesource.com/1222992
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#590992}
[modify] https://crrev.com/840917d42bdd5966e442f3bc1bbfcc2c06cd3b41/third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-gen-property-trees
[add] https://crrev.com/840917d42bdd5966e442f3bc1bbfcc2c06cd3b41/third_party/WebKit/LayoutTests/flag-specific/enable-blink-gen-property-trees/compositing/scrollbars/nested-overlay-scrollbars-expected.png
[modify] https://crrev.com/840917d42bdd5966e442f3bc1bbfcc2c06cd3b41/third_party/WebKit/LayoutTests/flag-specific/enable-blink-gen-property-trees/compositing/squashing/no-squashing-into-another-clip-layer-expected.txt