Add missing semantics flag for embedder (flutter#25932)

Add in flags kFlutterSemanticsFlagIsSlider and kFlutterSemanticsFlagIsKeyboardKey
from SemanticsFlags.

Author: yyzhong-g

shell/platform/embedder/embedder.h

@@ -210,6 +210,10 @@ typedef enum {
   kFlutterSemanticsFlagIsFocusable = 1 << 21,
   /// Whether the semantics node represents a link.
   kFlutterSemanticsFlagIsLink = 1 << 22,
+  /// Whether the semantics node represents a slider.
+  kFlutterSemanticsFlagIsSlider = 1 << 23,
+  /// Whether the semantics node represents a keyboard key.
+  kFlutterSemanticsFlagIsKeyboardKey = 1 << 24,
 } FlutterSemanticsFlag;
 
 typedef enum {