The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/4bd9033d09cb618c71d5714222ba1c9029d168fa

commit 4bd9033d09cb618c71d5714222ba1c9029d168fa
Author: Sandra Sun <sunyunjia@chromium.org>
Date: Fri Aug 25 16:23:12 2017

Apply root element's scroll-behavior to the viewport.

According to the spec
https://www.w3.org/TR/cssom-view-1/#propdef-scroll-behavior, we should apply
root element's scroll-behavior to viewport.

Bug:  758723 
Change-Id: I03927620fa5a61e4d42c5094b6903abfe2883123
Reviewed-on: https://chromium-review.googlesource.com/634326
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Sandra Sun <sunyunjia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#497433}
[modify] https://crrev.com/4bd9033d09cb618c71d5714222ba1c9029d168fa/third_party/WebKit/Source/core/dom/Document.cpp
[modify] https://crrev.com/4bd9033d09cb618c71d5714222ba1c9029d168fa/third_party/WebKit/Source/core/page/scrolling/SmoothScrollTest.cpp