Add missing kHasImplicitScrolling enum value

This brings the Dart and C++ semantics flag enums back in sync.

In #5941, implicit scrolling support was added to SemanticsFlag in
dart:ui, and to the Android embedder, but not to the SemanticsFlags enum
on the C++ side.

Author: cbracken

lib/ui/semantics.dart

@@ -264,8 +264,9 @@ class SemanticsAction {
 
 /// A Boolean value that can be associated with a semantics node.
 //
-// When changes are made to this class, the equivalent APIs in each of the
-// embedders *must* be updated.
+// When changes are made to this class, the equivalent APIs in
+// `lib/ui/semantics/semantics_node.h` and in each of the embedders *must* be
+// updated.
 class SemanticsFlag {
   static const int _kHasCheckedStateIndex = 1 << 0;
   static const int _kIsCheckedIndex = 1 << 1;

lib/ui/semantics/semantics_node.h

@@ -65,6 +65,7 @@ enum class SemanticsFlags : int32_t {
   kIsLiveRegion = 1 << 15,
   kHasToggledState = 1 << 16,
   kIsToggled = 1 << 17,
+  kHasImplicitScrolling = 1 << 18,
 };
 
 struct SemanticsNode {

shell/platform/embedder/embedder.h

@@ -141,6 +141,14 @@ typedef enum {
   kFlutterSemanticsFlagHasToggledState = 1 << 16,
   // If true, the semantics node is "on". If false, the semantics node is "off".
   kFlutterSemanticsFlagIsToggled = 1 << 17,
+  // Whether the platform can scroll the semantics node when the user attempts
+  // to move focus to an offscreen child.
+  //
+  // For example, a ListView widget has implicit scrolling so that users can
+  // easily move to the next visible set of children. A TabBar widget does not
+  // have implicit scrolling, so that users can navigate into the tab body when
+  // reaching the end of the tab bar.
+  kFlutterSemanticsFlagHasImplicitScrolling = 1 << 18,
 } FlutterSemanticsFlag;
 
 typedef enum {