GSB uses delta_hints to calculate scrolling chain.

cc/trees/layer_tree_host_impl_unittest.cc

 // Copyright 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/trees/layer_tree_host_impl.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 #include <cmath>
@@ skipping 6227 matching lines @@
     gfx::Vector2d gesture_scroll_delta(10, 0);
     EXPECT_EQ(InputHandler::SCROLL_ON_IMPL_THREAD,
               host_impl_
                   ->ScrollBegin(BeginState(gfx::Point(1, 1)).get(),
                                 InputHandler::TOUCHSCREEN)
                   .thread);
     host_impl_->ScrollBy(UpdateState(gfx::Point(), gesture_scroll_delta).get());
     host_impl_->ScrollEnd(EndState().get());
 
     // The child layer shouldn't have scrolled.
-    gfx::Vector2d expected_scroll_delta(
-        0, -gesture_scroll_delta.x() *
-               std::sin(MathUtil::Deg2Rad(child_layer_angle)));
     std::unique_ptr<ScrollAndScaleSet> scroll_info =
         host_impl_->ProcessScrollDeltas();
-    EXPECT_TRUE(ScrollInfoContains(*scroll_info.get(), child_scroll_id,
-                                   expected_scroll_delta));
+    ExpectNone(*scroll_info.get(), child_scroll_id);

[bokan, 2017/06/19 21:43:17]

What's this change about? The comment above says the layer shouldn't have
scrolled but it used to check that it does? Was the test before incorrect? Does
your patch fix the behavior?

[sahel, 2017/06/22 16:34:17]

As we discussed the comment is incorrect, I confirmed it with tdresser@ as well.
The child layer should scroll, so I left the test expectation as it is and
corrected the comment. I had to change layer_tree_host_impl.cc to take
transformations into account in canConsumeDelta function.  

[bokan, 2017/06/22 21:44:37]

Acknowledged.

 
     // The root scroll layer shouldn't have scrolled.
     ExpectNone(*scroll_info.get(), scroll_layer->element_id());
   }
 }
 
 TEST_F(LayerTreeHostImplTest, ScrollPerspectiveTransformedLayer) {
   // When scrolling an element with perspective, the distance scrolled
   // depends on the point at which the scroll begins.
   LayerImpl* scroll_layer = SetupScrollAndContentsLayers(gfx::Size(100, 100));
@@ skipping 4651 matching lines @@
                    scrolling_layer->CurrentScrollOffset());
   EXPECT_EQ(nullptr, host_impl_->CurrentlyScrollingNode());
   host_impl_->DidFinishImplFrame();
 }
 
 TEST_F(LayerTreeHostImplTest, SecondScrollAnimatedBeginNotIgnored) {
   const gfx::Size content_size(1000, 1000);
   const gfx::Size viewport_size(50, 100);
   CreateBasicVirtualViewportLayers(viewport_size, content_size);
 
-  EXPECT_EQ(InputHandler::SCROLL_ON_IMPL_THREAD,
-            host_impl_->ScrollAnimatedBegin(gfx::Point()).thread);
+  EXPECT_EQ(
+      InputHandler::SCROLL_ON_IMPL_THREAD,
+      host_impl_->ScrollAnimatedBegin(BeginState(gfx::Point()).get()).thread);
 
   // The second ScrollAnimatedBegin should not get ignored.
-  EXPECT_EQ(InputHandler::SCROLL_ON_IMPL_THREAD,
-            host_impl_->ScrollAnimatedBegin(gfx::Point()).thread);
+  EXPECT_EQ(
+      InputHandler::SCROLL_ON_IMPL_THREAD,
+      host_impl_->ScrollAnimatedBegin(BeginState(gfx::Point()).get()).thread);
 }
 
 // Verfify that a smooth scroll animation doesn't jump when UpdateTarget gets
 // called before the animation is started.
 TEST_F(LayerTreeHostImplTest, AnimatedScrollUpdateTargetBeforeStarting) {
   const gfx::Size content_size(1000, 1000);
   const gfx::Size viewport_size(50, 100);
   CreateBasicVirtualViewportLayers(viewport_size, content_size);
 
   DrawFrame();
@@ skipping 1701 matching lines @@
   // layer should be prioritized over the hidden layer.
   hidden_layer->set_contributes_to_drawn_render_surface(false);
   hidden_layer->set_raster_even_if_not_drawn(true);
   queue = host_impl_->BuildRasterQueue(TreePriority::SMOOTHNESS_TAKES_PRIORITY,
                                        RasterTilePriorityQueue::Type::ALL);
   EXPECT_EQ(queue->Top().tile()->layer_id(), 3);
 }
 
 }  // namespace
 }  // namespace cc