Maybe I should use overscroll_elasticity_transform_node_ instead of transform_parent here.

https://cs.chromium.org/chromium/src/third_party/blink/renderer/core/frame/visual_viewport.cc?rcl=b47bb9f8db4b3275d13cc85b6858ae3be1fbdfb7&l=219

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/6956df955277449a2f6b8eea76c52b908b13a9ce

commit 6956df955277449a2f6b8eea76c52b908b13a9ce
Author: chaopeng <chaopeng@chromium.org>
Date: Thu Oct 11 21:45:29 2018

[blink-gen-property-tree] root scrollbar should use root transform node

This issue is because we set the transform node of root scrollbar to
the transform node of root scroller which is under the overscroll
transform node.

Bug:  893155 
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux-blink-gen-property-trees;luci.chromium.try:linux_layout_tests_layout_ng;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I0bee661a049076f41ebef7950cd75a9e924db927
Reviewed-on: https://chromium-review.googlesource.com/c/1274190
Reviewed-by: Philip Rogers <pdr@chromium.org>
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598961}
[modify] https://crrev.com/6956df955277449a2f6b8eea76c52b908b13a9ce/third_party/blink/renderer/core/paint/compositing/compositing_layer_property_updater.cc
[modify] https://crrev.com/6956df955277449a2f6b8eea76c52b908b13a9ce/third_party/blink/renderer/core/paint/compositing/compositing_layer_property_updater_test.cc