Add a setting to disable threaded scrolling

Source/web/tests/ScrollingCoordinatorChromiumTest.cpp

Index: Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
diff --git a/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp b/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
index a59135a02c4e839b932fc56f57b63195a8a90d47..d8819361cf020a05b22d6d69fc98bf3a5bfab1e5 100644
--- a/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
+++ b/Source/web/tests/ScrollingCoordinatorChromiumTest.cpp
@@ -124,6 +124,24 @@ TEST_F(ScrollingCoordinatorChromiumTest, fastScrollingByDefault)
     ASSERT_FALSE(rootScrollLayer->haveWheelEventHandlers());
 }
 
+TEST_F(ScrollingCoordinatorChromiumTest, mainScrollingWhenFastScrollingDisabled)
+{
+    navigateTo("about:blank");
+    webViewImpl()->settings()->setAcceleratedScrollingEnabled(false);
+    forceFullCompositingUpdate();
+
+    // Make sure the scrolling coordinator is active.
+    FrameView* frameView = frame()->view();
+    Page* page = frame()->page();
+    ASSERT_TRUE(page->scrollingCoordinator());
+    ASSERT_TRUE(page->scrollingCoordinator()->coordinatesScrollingForFrameView(frameView));
+
+    // Main scrolling should be enabled with the setting override.
+    WebLayer* rootScrollLayer = getRootScrollLayer();
+    ASSERT_TRUE(rootScrollLayer->scrollable());
+    ASSERT_TRUE(rootScrollLayer->shouldScrollOnMainThread());
+}
+
 static WebLayer* webLayerFromElement(Element* element)
 {
     if (!element)