Remove status:experimental for ImplicitRootScroller and enable it for specific unit tests

This is based on the answer in
https://docs.google.com/spreadsheets/d/1FeigUaKrTBKPx36U4v0hJMlQZ_DAq-tYlSiG-LvhWLg/edit.

This prepares for letting blink feature status override chromium feature
status.

Bug: 1121839
Change-Id: Ia44d4cc167319cac267820e30bd5388836327e2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377996Reviewed-by: David Bokan <bokan@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#801921}

third_party/blink/renderer/core/exported/web_frame_test.cc

@@ -12003,6 +12003,8 @@ TEST_F(WebFrameSimTest, TestScrollFocusedEditableElementIntoView) {
 // Ensures scrolling a focused editable text into view that's located in the
 // root scroller works by scrolling the root scroller.
 TEST_F(WebFrameSimTest, TestScrollFocusedEditableInRootScroller) {
+  ScopedImplicitRootScrollerForTest implicit_root_scroller(true);
+
   WebView().MainFrameWidget()->Resize(WebSize(500, 300));
   WebView().SetDefaultPageScaleLimits(1.f, 4);
   WebView().EnableFakePageScaleAnimationForTesting(true);

third_party/blink/renderer/core/page/scrolling/scroll_into_view_test.cc

@@ -535,6 +535,8 @@ TEST_F(ScrollIntoViewTest, ApplyRootElementScrollBehaviorToViewport) {
 // This test ensures the stop-at-layout viewport option works correctly when a
 // non-default root scroller is set as the layout viewport.
 TEST_F(ScrollIntoViewTest, StopAtLayoutViewportOption) {
+  ScopedImplicitRootScrollerForTest implicit_root_scroller(true);
+
   v8::HandleScope HandleScope(v8::Isolate::GetCurrent());
   WebView().MainFrameWidget()->Resize(WebSize(800, 600));
   SimRequest request("https://example.com/test.html", "text/html");

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -919,7 +919,6 @@
     },
     {
       name: "ImplicitRootScroller",
-      status: "experimental",
       settable_from_internals: true,
     },
     {