Allow turning on selection painting without selection gaps via window.internals.

Source/core/testing/Internals.cpp

Index: Source/core/testing/Internals.cpp
diff --git a/Source/core/testing/Internals.cpp b/Source/core/testing/Internals.cpp
index 34a34af9dd054a0d727e68bffc30aa274472b138..fd5efade26661a4fe749f0ff6833c317dd2b1578 100644
--- a/Source/core/testing/Internals.cpp
+++ b/Source/core/testing/Internals.cpp
@@ -2475,6 +2475,11 @@ void Internals::setCapsLockState(bool enabled)
         PlatformKeyboardEvent::OverrideCapsLockState::On : PlatformKeyboardEvent::OverrideCapsLockState::Off);
 }
 
+void Internals::setSelectionPaintingWithoutSelectionGaps(bool enabled)
+{
+    RuntimeEnabledFeatures::setSelectionPaintingWithoutSelectionGapsEnabled(enabled);
+}
+
 bool Internals::setScrollbarVisibilityInScrollableArea(Node* node, bool visible)
 {
     LayoutObject* layoutObject = node->layoutObject();