[LayoutNG] Fix ScrollbarAppearanceTest.NativeScrollbarChangeToMobileByEmulator

As above.

This iterates over all the scrollable areas, and marks their respective
layout objects as requiring layout.

This is required as LayoutNG has caches more items than in the existing
LayoutNG, and nothing marks the layout_box with the scrollbar as needing
layout.

This is similar to the kPaintChange, which invalidates paint all of paint
when something like high-contrast mode is enabled.

Bug: 922456
Change-Id: Ie65b14872a832b73b197247c06ce77727d8e6aec
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1476811
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: Morten Stenshorne <mstensho@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#638247}

third_party/blink/renderer/core/frame/settings.json5

@@ -574,6 +574,7 @@
     {
       name: "forceAndroidOverlayScrollbar",
       initial: false,
+      invalidate: "ScrollbarLayout",
     },
 
     // Set the timeout seconds of the network-quiet timers in IdlenessDetector.

third_party/blink/renderer/core/frame/settings_delegate.h

@@ -68,6 +68,7 @@ class CORE_EXPORT SettingsDelegate {
     kPluginsChange,
     kHighlightAdsChange,
     kPaintChange,
+    kScrollbarLayoutChange,
   };
 
   virtual void SettingsChanged(ChangeType) = 0;

third_party/blink/renderer/core/layout/scrollbars_test.cc

@@ -1432,10 +1432,6 @@ TEST_P(ScrollbarAppearanceTest,
 #else
 TEST_P(ScrollbarAppearanceTest, NativeScrollbarChangeToMobileByEmulator) {
 #endif
-  // TODO(crbug.com/934634): This test fails with LayoutNG.
-  if (RuntimeEnabledFeatures::LayoutNGEnabled())
-    return;
-
   ScopedTestingPlatformSupport<ScrollbarTestingPlatformSupport> platform;
   bool use_overlay_scrollbar = GetParam();
 

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

@@ -69,6 +69,7 @@
 #include "third_party/blink/renderer/core/page/scrolling/top_document_root_scroller_controller.h"
 #include "third_party/blink/renderer/core/page/spatial_navigation_controller.h"
 #include "third_party/blink/renderer/core/page/validation_message_client_impl.h"
+#include "third_party/blink/renderer/core/paint/paint_layer_scrollable_area.h"
 #include "third_party/blink/renderer/core/probe/core_probes.h"
 #include "third_party/blink/renderer/core/scroll/scrollbar_theme.h"
 #include "third_party/blink/renderer/core/scroll/scrollbar_theme_overlay.h"
@@ -664,7 +665,7 @@ void Page::SettingsChanged(SettingsDelegate::ChangeType change_type) {
       }
       break;
     }
-    case SettingsDelegate::kPaintChange:
+    case SettingsDelegate::kPaintChange: {
       for (Frame* frame = MainFrame(); frame;
            frame = frame->Tree().TraverseNext()) {
         auto* local_frame = DynamicTo<LocalFrame>(frame);
@@ -674,6 +675,28 @@ void Page::SettingsChanged(SettingsDelegate::ChangeType change_type) {
           view->InvalidatePaintForViewAndCompositedLayers();
       }
       break;
+    }
+    case SettingsDelegate::kScrollbarLayoutChange: {
+      for (Frame* frame = MainFrame(); frame;
+           frame = frame->Tree().TraverseNext()) {
+        auto* local_frame = DynamicTo<LocalFrame>(frame);
+        if (!local_frame)
+          continue;
+        // Iterate through all of the scrollable areas and mark their layout
+        // objects for layout.
+        if (LocalFrameView* view = local_frame->View()) {
+          if (const auto* scrollable_areas = view->ScrollableAreas()) {
+            for (const auto& scrollable_area : *scrollable_areas) {
+              if (auto* layout_box = scrollable_area->GetLayoutBox()) {
+                layout_box->SetNeedsLayout(
+                    layout_invalidation_reason::kScrollbarChanged);
+              }
+            }
+          }
+        }
+      }
+      break;
+    }
   }
 }