Disable kShelfScrollable feature by default.

This feature is not launching in M79.

Temporarily disabled two tricky tests, filed bugs to fix:
http://crbug.com/1015638
http://crbug.com/1015641

Bug: 973481
Test: ash_unittests
Change-Id: I17b2bda3fd383f76dbdf81f86ab638604d923582
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1865712Reviewed-by: Manu Cornet <manucornet@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Vladislav Kaznacheev <kaznacheev@chromium.org>
Cr-Commit-Position: refs/heads/master@{#707369}

ash/shelf/shelf_view_unittest.cc

@@ -1698,7 +1698,7 @@ TEST_F(ShelfViewTest, OverflowBubbleSize) {
             overflow_shelf_view->GetPreferredSize().width());
 }
 
-TEST_F(ShelfViewTest, OverflowShelfColorIsDerivedFromWallpaper) {
+TEST_F(ShelfViewTest, DISABLED_OverflowShelfColorIsDerivedFromWallpaper) {
   // No overflow bubble when scrollable shelf enabled.
   // TODO(https://crbug.com/1002576): revisit when scrollable shelf is launched.
   if (chromeos::switches::ShouldShowScrollableShelf())
@@ -1905,7 +1905,7 @@ TEST_F(ShelfViewTest, CheckRipOffFromLeftShelfAlignmentWithMultiMonitor) {
 
 // Checks various drag and drop operations from OverflowBubble to Shelf, and
 // vice versa.
-TEST_F(ShelfViewTest, CheckDragAndDropFromShelfToOtherShelf) {
+TEST_F(ShelfViewTest, DISABLED_CheckDragAndDropFromShelfToOtherShelf) {
   // No overflow bubble when scrollable shelf enabled.
   // TODO(https://crbug.com/1002576): revisit when scrollable shelf is launched.
   if (chromeos::switches::ShouldShowScrollableShelf())

chromeos/constants/chromeos_features.cc

@@ -183,7 +183,7 @@ const base::Feature kSessionManagerLongKillTimeout{
 
 // Enables or disables the scrollable shelf.
 const base::Feature kShelfScrollable{"ShelfScrollable",
-                                     base::FEATURE_ENABLED_BY_DEFAULT};
+                                     base::FEATURE_DISABLED_BY_DEFAULT};
 
 // Enables or disables the shelf hotseat.
 const base::Feature kShelfHotseat{"ShelfHotseat",