Adds scroll position/scale emulation to DevTools.

cc/trees/layer_tree_host_impl_unittest.cc

Index: cc/trees/layer_tree_host_impl_unittest.cc
diff --git a/cc/trees/layer_tree_host_impl_unittest.cc b/cc/trees/layer_tree_host_impl_unittest.cc
index 925993108a384d217951602c1ccae3945ff3af7a..09e34e74328d0f55a140d7d8bf3eb7663b32509a 100644
--- a/cc/trees/layer_tree_host_impl_unittest.cc
+++ b/cc/trees/layer_tree_host_impl_unittest.cc
@@ -6270,6 +6270,42 @@ TEST_F(LayerTreeHostImplTest, NoOverscrollWhenNotAtEdge) {
   }
 }
 
+TEST_F(LayerTreeHostImplTest, OverscrollOnMainThread) {
+  InputHandlerScrollResult scroll_result;
+  LayerTreeSettings settings = DefaultSettings();
+  CreateHostImpl(settings, CreateOutputSurface());
+
+  LayerImpl* scroll_layer = SetupScrollAndContentsLayers(gfx::Size(50, 50));
+  scroll_layer->set_main_thread_scrolling_reasons(
+      MainThreadScrollingReason::kThreadedScrollingDisabled);
+
+  LayerImpl* clip_layer =
+      scroll_layer->test_properties()->parent->test_properties()->parent;
+  clip_layer->SetBounds(gfx::Size(50, 50));
+  host_impl_->active_tree()->BuildPropertyTreesForTesting();
+  host_impl_->SetViewportSize(gfx::Size(50, 50));
+  host_impl_->active_tree()->PushPageScaleFromMainThread(1.f, 0.5f, 4.f);
+  DrawFrame();
+
+  // Overscroll initiated outside layers will be handled by the main thread.
+  EXPECT_EQ(nullptr, host_impl_->active_tree()->FindLayerThatIsHitByPoint(
+                         gfx::PointF(0, 60)));
+  EXPECT_EQ(InputHandler::SCROLL_ON_MAIN_THREAD,
+            host_impl_
+                ->ScrollBegin(BeginState(gfx::Point(0, 60)).get(),
+                              InputHandler::WHEEL)
+                .thread);
+
+  // Overscroll initiated inside layers will be handled by the main thread.
+  EXPECT_NE(nullptr, host_impl_->active_tree()->FindLayerThatIsHitByPoint(
+                         gfx::PointF(0, 0)));
+  EXPECT_EQ(
+      InputHandler::SCROLL_ON_MAIN_THREAD,
+      host_impl_
+          ->ScrollBegin(BeginState(gfx::Point(0, 0)).get(), InputHandler::WHEEL)
+          .thread);
+}
+
 class BlendStateCheckLayer : public LayerImpl {
  public:
   static std::unique_ptr<LayerImpl> Create(