Fix smooth scroll jump when switching scroll handling between MT and CC

cc/layers/layer_unittest.cc

Index: cc/layers/layer_unittest.cc
diff --git a/cc/layers/layer_unittest.cc b/cc/layers/layer_unittest.cc
index afc6c359397f36004d90ecb1cd7390ba5b6116e6..558df01694e1b6cb94b03fe05db2838e25b1e589 100644
--- a/cc/layers/layer_unittest.cc
+++ b/cc/layers/layer_unittest.cc
@@ -928,6 +928,48 @@ TEST_F(LayerTest, CheckSetNeedsDisplayCausesCorrectBehavior) {
   EXPECT_TRUE(test_layer->NeedsDisplayForTesting());
 }
 
+TEST_F(LayerTest, TestSettingMainThreadScrollingReason) {
+  scoped_refptr<Layer> test_layer = Layer::Create(layer_settings_);
+  EXPECT_SET_NEEDS_FULL_TREE_SYNC(1,
+                                  layer_tree_host_->SetRootLayer(test_layer));
+  EXPECT_SET_NEEDS_COMMIT(1, test_layer->SetIsDrawable(true));
+
+  // sanity check of initial test condition
+  EXPECT_FALSE(test_layer->NeedsDisplayForTesting());
+
+  uint32_t reasons = 0, reasons_to_clear = 0, reasons_after_clearing = 0;
+  reasons |= MainThreadScrollingReason::kEventHandlers;
+  reasons |= MainThreadScrollingReason::kContinuingMainThreadScroll;
+  reasons |= MainThreadScrollingReason::kScrollbarScrolling;
+
+  reasons_to_clear |= MainThreadScrollingReason::kContinuingMainThreadScroll;
+  reasons_to_clear |= MainThreadScrollingReason::kThreadedScrollingDisabled;
+
+  reasons_after_clearing |= MainThreadScrollingReason::kEventHandlers;
+  reasons_after_clearing |= MainThreadScrollingReason::kScrollbarScrolling;
+
+  // Check that the reasons are added correctly.
+  EXPECT_SET_NEEDS_COMMIT(1, test_layer->AddMainThreadScrollingReasons(
+                                 MainThreadScrollingReason::kEventHandlers));
+  EXPECT_SET_NEEDS_COMMIT(
+      1, test_layer->AddMainThreadScrollingReasons(
+             MainThreadScrollingReason::kContinuingMainThreadScroll));
+  EXPECT_SET_NEEDS_COMMIT(1,
+                          test_layer->AddMainThreadScrollingReasons(
+                              MainThreadScrollingReason::kScrollbarScrolling));
+  EXPECT_EQ(reasons, test_layer->main_thread_scrolling_reasons());
+
+  // Check that the reasons can be selectively cleared.
+  EXPECT_SET_NEEDS_COMMIT(
+      1, test_layer->ClearMainThreadScrollingReasons(reasons_to_clear));
+  EXPECT_EQ(reasons_after_clearing,
+            test_layer->main_thread_scrolling_reasons());
+
+  // Check that all reasons can be cleared.
+  EXPECT_SET_NEEDS_COMMIT(1, test_layer->ClearMainThreadScrollingReasons());
+  EXPECT_EQ((uint32_t)0, test_layer->main_thread_scrolling_reasons());
+}
+
 TEST_F(LayerTest, CheckPropertyChangeCausesCorrectBehavior) {
   scoped_refptr<Layer> test_layer = Layer::Create(layer_settings_);
   EXPECT_SET_NEEDS_FULL_TREE_SYNC(