The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/bf23241073ef806810f2aa90bbb153b639239c81

commit bf23241073ef806810f2aa90bbb153b639239c81
Author: David Bokan <bokan@chromium.org>
Date: Wed Aug 15 21:17:40 2018

[BlinkGenPropertyTrees] Implement pos:fixed URL bar adjustment

This CL implements the "viewport bounds delta" for position: fixed
elements. When the URL bar is hidden, the renderer is only resized
when the user finishes scrolling. This means that while the scroll
is on-going and the URL bar is hiding, the renderer is actually
smaller than the visible area - we're revealing new content outside
the renderer's bounds.

Since position: fixed elements are positioned relative to the top-
left corner of a layer, a bottom-fixed element would appear to
detach from the bottom of the screen and move upwards as the URL
bar hides. CC uses some slight-of-hand to prevent this by adding a
"viewport bounds delta" to the scrolling and clipping bounds of the
viewport layers. This allows the newly revealed content to be seen
and keeps bottom-fixed elements static until the renderer itself is
resized and the bounds delta can be cleared.

This CL does the following things:

1) Update Blink to set the correct paint property tree state on
bottom-fixed objects so that the bounds delta is correctly applied
to them.

2) Modify the compositor to apply the bounds delta to the property
trees directly, rather than using layers which may no longer exist
in BGPT mode

3) Explicitly clamp the scroll offset on pending tree activation.
LayerTreeHostImplTest, ClampingAfterActivation was failing after
the early return was added to UpdateViewportContainerSizes. We
were relying on the viewport scroll anchor to clamp the scroll
offsets. Better to do this explicitly.

The marked test is currently crashing but tests this behavior. I've
confirmed that ignoring the crash causes the test to fail as
expected and applying this patch fixes it.

TEST=virtual/android/url-bar/bottom-fixed-adjusted-when-showing-url-bar.html

Bug:  850135 
Cq-Include-Trybots: luci.chromium.try:linux-blink-gen-property-trees;luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_layout_tests_slimming_paint_v2;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I3a2da2648b47c2ccc0baf2876da12c228213dbe6
Reviewed-on: https://chromium-review.googlesource.com/1173441
Commit-Queue: David Bokan <bokan@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#583398}
[modify] https://crrev.com/bf23241073ef806810f2aa90bbb153b639239c81/cc/layers/layer_impl.h
[modify] https://crrev.com/bf23241073ef806810f2aa90bbb153b639239c81/cc/trees/layer_tree_host_impl.cc
[modify] https://crrev.com/bf23241073ef806810f2aa90bbb153b639239c81/third_party/WebKit/LayoutTests/FlagExpectations/enable-blink-gen-property-trees
[modify] https://crrev.com/bf23241073ef806810f2aa90bbb153b639239c81/third_party/blink/renderer/core/paint/paint_property_tree_builder.cc
[modify] https://crrev.com/bf23241073ef806810f2aa90bbb153b639239c81/third_party/blink/renderer/platform/graphics/compositing/property_tree_manager.cc
[modify] https://crrev.com/bf23241073ef806810f2aa90bbb153b639239c81/third_party/blink/renderer/platform/graphics/paint/transform_paint_property_node.h