The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/5b21b4ef35554c4e5c89dd04be66f011978bdf16

commit 5b21b4ef35554c4e5c89dd04be66f011978bdf16
Author: Sandra Sun <sunyunjia@chromium.org>
Date: Thu May 17 23:28:36 2018

Use visual scroll offset for snapping.

Currently, snap points only consider the scroll offset of the
LayoutViewport(outer viewport scroll layer in cc), ignoring the scroll
offset of the VisualViewport(inner viewport scroll layer in cc). This
would confuse the user as they enlarge the page.

This patch makes sure that the coordinate for snapping is based on
visual viewport, and the snapping behavior scrolls visual viewport when
necessary.

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I29ad271b04ac3fffec733f764e9168bc08150507

Bug:  839971 
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I29ad271b04ac3fffec733f764e9168bc08150507
Reviewed-on: https://chromium-review.googlesource.com/1033351
Reviewed-by: Dave Tapuska <dtapuska@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Sandra Sun <sunyunjia@chromium.org>
Cr-Commit-Position: refs/heads/master@{#559729}
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/cc/input/input_handler.h
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/cc/layers/viewport.cc
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/cc/layers/viewport.h
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/cc/trees/layer_tree_host_impl.cc
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/cc/trees/layer_tree_host_impl.h
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/cc/trees/layer_tree_host_impl_unittest.cc
[add] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/third_party/WebKit/LayoutTests/fast/scroll-snap/snap-scrolls-visual-viewport.html
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/third_party/blink/renderer/core/frame/root_frame_viewport.cc
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/third_party/blink/renderer/core/frame/root_frame_viewport.h
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/third_party/blink/renderer/core/page/scrolling/snap_coordinator.cc
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/third_party/blink/renderer/platform/scroll/scrollable_area.h
[modify] https://crrev.com/5b21b4ef35554c4e5c89dd04be66f011978bdf16/ui/events/blink/input_handler_proxy.cc