This seems related to: https://bugs.chromium.org/p/chromium/issues/detail?id=233853

We shouldn't be doing that fishy business grabbing the first child of the root scroll layer.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/cae4f1070f808d4c1f8ebce25901d653191be3bc

commit cae4f1070f808d4c1f8ebce25901d653191be3bc
Author: sunxd <sunxd@chromium.org>
Date: Wed Apr 06 15:04:15 2016

cc: Use scroll layer bounds as scrollable area size

LTI::ScrollableSize suspiciously took the bounds of the first child
of the scrollable layer as the scrollable area size because of some
racing condition in blink.

Now we are not sure if the racing condition still exists. This CL
tries to use the bounds of the scrollable layer itself and see if
it breaks anything.

BUG= 233853 ,  598381 
CQ_INCLUDE_TRYBOTS=tryserver.blink:linux_blink_rel

Review URL: https://codereview.chromium.org/1862623003

Cr-Commit-Position: refs/heads/master@{#385464}

[modify] https://crrev.com/cae4f1070f808d4c1f8ebce25901d653191be3bc/cc/trees/layer_tree_impl.cc

Since now we agreed to make the change to directly use the scrollable layer's bounds, we will not do anything about the hierarchy here.

Unless that change breaks anything and we need to use the child's bounds anyway, we will move back to this bug again. So I'm merging this to  crbug.com/233853 .