Add experimental Support for DOM3 KeyboardEvent key value

Source/platform/PlatformKeyboardEvent.h

Index: Source/platform/PlatformKeyboardEvent.h
diff --git a/Source/platform/PlatformKeyboardEvent.h b/Source/platform/PlatformKeyboardEvent.h
index 0edb3a5d4ac95e989e62abd30757876be4da01c1..c024a03f6b2311e5a6c6b27fcf3468a43aa9f63a 100644
--- a/Source/platform/PlatformKeyboardEvent.h
+++ b/Source/platform/PlatformKeyboardEvent.h
@@ -46,12 +46,13 @@ public:
     {
     }
 
-    PlatformKeyboardEvent(Type type, const String& text, const String& unmodifiedText, const String& keyIdentifier, const String& code, int windowsVirtualKeyCode, int nativeVirtualKeyCode, bool isAutoRepeat, bool isKeypad, bool isSystemKey, Modifiers modifiers, double timestamp)
+    PlatformKeyboardEvent(Type type, const String& text, const String& unmodifiedText, const String& keyIdentifier, const String& code, const String& key, int windowsVirtualKeyCode, int nativeVirtualKeyCode, bool isAutoRepeat, bool isKeypad, bool isSystemKey, Modifiers modifiers, double timestamp)
         : PlatformEvent(type, modifiers, timestamp)
         , m_text(text)
         , m_unmodifiedText(unmodifiedText)
         , m_keyIdentifier(keyIdentifier)
         , m_code(code)
+        , m_key(key)
         , m_windowsVirtualKeyCode(windowsVirtualKeyCode)
         , m_nativeVirtualKeyCode(nativeVirtualKeyCode)
         , m_autoRepeat(isAutoRepeat)
@@ -78,6 +79,7 @@ public:
     String keyIdentifier() const { return m_keyIdentifier; }
 
     String code() const { return m_code; }
+    String key() const { return m_key; }
 
     // Most compatible Windows virtual key code associated with the event.
     // Zero for Char events.
@@ -97,6 +99,7 @@ protected:
     String m_unmodifiedText;
     String m_keyIdentifier;
     String m_code;
+    String m_key;
     int m_windowsVirtualKeyCode;
     int m_nativeVirtualKeyCode;
     bool m_autoRepeat;