Ignore root scrolling when layer_list in active_tree is empty

This crash is because we have root layer is null and we don't handle it properly.
Root layer is null can happen if a scroll occurs before the root layer is attached.

In this patch, we return SCROLL_IGNORED with kNoScrollingLayer reason, same as what
we did in ScrollBegin.

Bug: 895817
Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;master.tryserver.blink:linux_trusty_blink_rel
Change-Id: I9937ae2d494f4466018a3ee9a35e787e5e745bd6
Reviewed-on: https://chromium-review.googlesource.com/c/1289437Reviewed-by: David Bokan <bokan@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Commit-Queue: Jianpeng Chao <chaopeng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#601345}

cc/trees/layer_tree_host_impl_unittest.cc

@@ -1022,6 +1022,20 @@ TEST_F(CommitToPendingTreeLayerTreeHostImplTest,
       "Compositing.Renderer.GPUMemoryForTilingsInKb", 1);
 }
 
+TEST_F(LayerTreeHostImplTest, ScrollBeforeRootLayerAttached) {
+  InputHandler::ScrollStatus status = host_impl_->ScrollBegin(
+      BeginState(gfx::Point()).get(), InputHandler::WHEEL);
+  EXPECT_EQ(InputHandler::SCROLL_IGNORED, status.thread);
+  EXPECT_EQ(MainThreadScrollingReason::kNoScrollingLayer,
+            status.main_thread_scrolling_reasons);
+
+  status = host_impl_->RootScrollBegin(BeginState(gfx::Point()).get(),
+                                       InputHandler::WHEEL);
+  EXPECT_EQ(InputHandler::SCROLL_IGNORED, status.thread);
+  EXPECT_EQ(MainThreadScrollingReason::kNoScrollingLayer,
+            status.main_thread_scrolling_reasons);
+}
+
 TEST_F(LayerTreeHostImplTest, ScrollRootCallsCommitAndRedraw) {
   SetupScrollAndContentsLayers(gfx::Size(100, 100));
   host_impl_->active_tree()->BuildPropertyTreesForTesting();

cc/trees/layer_tree_impl.cc

@@ -2116,8 +2116,11 @@ struct FindScrollingLayerOrScrollbarFunctor {
 
 LayerImpl* LayerTreeImpl::FindFirstScrollingLayerOrScrollbarThatIsHitByPoint(
     const gfx::PointF& screen_space_point) {
+  if (layer_list_.empty())
+    return nullptr;
+
   FindClosestMatchingLayerState state;
-  LayerImpl* root_layer = layer_list_.empty() ? nullptr : layer_list_[0];
+  LayerImpl* root_layer = layer_list_[0];
   FindClosestMatchingLayer(screen_space_point, root_layer,
                            FindScrollingLayerOrScrollbarFunctor(), &state);
   return state.closest_match;