Fix for flaky test (scrollbar-button-gesture-target.html).

The test fails because it tries to dereference a raw pointer which
points to layer that was destroyed. The fix is to set the pointer
to null and add checks in scrollbar_controller.cc

Bug: 1002828, 987115
Change-Id: I46efc7e35ee62bb36020a42aafbd75998728017c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1854534
Commit-Queue: Rahul Arakeri <arakeri@microsoft.com>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706170}

cc/input/scrollbar_controller.cc

@@ -402,6 +402,15 @@ float ScrollbarController::GetScrollerToScrollbarRatio() {
   return scaled_scroller_to_scrollbar_ratio;
 }
 
+void ScrollbarController::DidUnregisterScrollbar(ElementId element_id) {
+  if (currently_captured_scrollbar_ &&
+      currently_captured_scrollbar_->scroll_element_id() == element_id)
+    currently_captured_scrollbar_ = nullptr;
+    drag_processed_for_current_frame_ = false;
+    drag_state_ = base::nullopt;
+    autoscroll_state_ = base::nullopt;
+}
+
 void ScrollbarController::RecomputeAutoscrollStateIfNeeded() {
   if (!autoscroll_state_.has_value())
     return;

cc/input/scrollbar_controller.h

@@ -38,6 +38,7 @@ class CC_EXPORT ScrollbarController {
   ScrollbarOrientation orientation() {
     return currently_captured_scrollbar_->orientation();
   }
+  void DidUnregisterScrollbar(ElementId element_id);
 
   void WillBeginImplFrame();
 

cc/trees/layer_tree_host_impl.cc

@@ -5317,6 +5317,10 @@ void LayerTreeHostImpl::RegisterScrollbarAnimationController(
                                                        scrollbar_opacity);
 }
 
+void LayerTreeHostImpl::UnregisterScrollbarLayer(ElementId element_id) const {
+  scrollbar_controller_->DidUnregisterScrollbar(element_id);
+}
+
 void LayerTreeHostImpl::UnregisterScrollbarAnimationController(
     ElementId scroll_element_id) {
   scrollbar_animation_controllers_.erase(scroll_element_id);

cc/trees/layer_tree_host_impl.h

@@ -373,6 +373,8 @@ class CC_EXPORT LayerTreeHostImpl : public InputHandler,
     return need_update_gpu_rasterization_status_;
   }
 
+  void UnregisterScrollbarLayer(ElementId element_id) const;
+
   // MutatorHostClient implementation.
   bool IsElementInPropertyTrees(ElementId element_id,
                                 ElementListType list_type) const override;

cc/trees/layer_tree_impl.cc

@@ -1919,6 +1919,7 @@ void LayerTreeImpl::UnregisterScrollbar(
     element_id_to_scrollbar_layer_ids_.erase(scroll_element_id);
     if (IsActiveTree()) {
       host_impl_->UnregisterScrollbarAnimationController(scroll_element_id);
+      host_impl_->UnregisterScrollbarLayer(scroll_element_id);
     }
   }
 }

third_party/blink/web_tests/TestExpectations

@@ -2538,13 +2538,7 @@ crbug.com/953847 [ Mac ] virtual/threaded/fast/scrolling/scrollbars/scroll-chain
 crbug.com/953847 [ Mac ] virtual/threaded/fast/scrolling/scrollbars/scrollbar-occluded-by-div.html [ Failure ]
 crbug.com/953847 [ Mac ] virtual/threaded/fast/scrolling/scrollbars/scrollbar-button-gesture-target.html [ Failure ]
 crbug.com/953847 [ Mac ] virtual/threaded/fast/scrolling/scrollbars/mouse-autoscrolling-on-scrollbar.html [ Failure Timeout ]
-
-# Compositor threaded scrollbar scrolls parent of subscroller.
-# Note: even when fixed will still fail on Mac until crbug.com/953847
-# is addressed. Uncomment below line when fixing crbug.com/965829.
-#crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/scrollbar-button-gesture-target.html [ Failure ]
-crbug.com/987115 [ Win ] virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/scrollbar-button-gesture-target.html [ Failure Crash ]
-crbug.com/965829 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/scrollbar-button-gesture-target.html [ Failure ]
+crbug.com/953847 [ Mac ] virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/scrollbar-button-gesture-target.html [ Failure ]
 
 # Some control characters still not visible
 crbug.com/893490 [ Mac ] external/wpt/css/css-text/white-space/control-chars-001.html [ Failure ]
@@ -5626,9 +5620,6 @@ crbug.com/1001936 external/wpt/css/css-masking/clip-path/clip-path-inline-001.ht
 crbug.com/1002527 [ Debug ] fast/text/large-text-composed-char.html [ Pass Crash ]
 crbug.com/1002686 [ Mac10.13 ] virtual/threaded/http/tests/devtools/tracing/timeline-style/timeline-recalculate-styles.js [ Failure ]
 
-# Sheriff 2019-09-11
-crbug.com/1002828 [ Linux ] virtual/compositor_threaded_scrollbar_scrolling/fast/scrolling/scrollbars/scrollbar-button-gesture-target.html [ Pass Failure Crash ]
-
 # Tests fail because of scrollbar-width property
 crbug.com/1003055 external/wpt/css/css-scroll-snap/scroll-target-align-001.html [ Failure ]
 crbug.com/1003055 external/wpt/css/css-scroll-snap/scroll-target-align-002.html [ Failure ]