The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c43aef99e7f29e1941d692eef1538116e1c2bdca

commit c43aef99e7f29e1941d692eef1538116e1c2bdca
Author: Philip Rogers <pdr@chromium.org>
Date: Thu Oct 19 19:23:56 2017

[root layer scrolling] Use correct scroll offset for dialogs

Dialog element centering used the frame view's scroll offset
but needed to use the scrollable area's scroll offset to work
with root layer scrolling.

Dialog centering is not very intuitive in that it does not
re-center when the frame is resized. For this reason, it is
not associated with the viewport constrained object set on
LocalFrameView. Because of this, it is safe to use the local
frame view's size instead of the (unavailable) scrollable
area size. A comment has been added about this discrepancy.

Bug:  776129 
Change-Id: I356e75799165cd3ee6202ce0ce54a638dacdfcae
Reviewed-on: https://chromium-review.googlesource.com/726851
Reviewed-by: Stefan Zager <szager@chromium.org>
Reviewed-by: Steve Kobes <skobes@chromium.org>
Commit-Queue: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#510156}
[modify] https://crrev.com/c43aef99e7f29e1941d692eef1538116e1c2bdca/third_party/WebKit/LayoutTests/FlagExpectations/root-layer-scrolls
[modify] https://crrev.com/c43aef99e7f29e1941d692eef1538116e1c2bdca/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp