Return AltGraph for right-Alt keydown/keyup, under layouts which use AltGraph.

ui/events/keycodes/platform_key_map_win.cc

Index: ui/events/keycodes/platform_key_map_win.cc
diff --git a/ui/events/keycodes/platform_key_map_win.cc b/ui/events/keycodes/platform_key_map_win.cc
index 439384c634941d9eb9017b047e3dd464860a9f9f..32b544e31ca8b72463fc8b75052ff91984cc3b5b 100644
--- a/ui/events/keycodes/platform_key_map_win.cc
+++ b/ui/events/keycodes/platform_key_map_win.cc
@@ -286,6 +286,12 @@ PlatformKeyMap::~PlatformKeyMap() {}
 
 DomKey PlatformKeyMap::DomKeyFromKeyboardCodeImpl(KeyboardCode key_code,
                                                   int flags) const {
+  // Windows expresses right-Alt as VKEY_MENU with the extended flag set.
+  // This key should generate AltGraph under layouts which use that modifier.
+  if (key_code == VKEY_MENU && (flags & EF_IS_EXTENDED_KEY) && has_alt_graph_) {
+    return DomKey::ALT_GRAPH;
+  }
+
   DomKey key = NonPrintableKeyboardCodeToDomKey(key_code, keyboard_layout_);
   if (key != DomKey::NONE)
     return key;
@@ -346,6 +352,7 @@ void PlatformKeyMap::UpdateLayout(HKL layout) {
   // TODO(chongz): Optimize layout switching (see crbug.com/587147).
   keyboard_layout_ = layout;
   printable_keycode_to_key_.clear();
+  has_alt_graph_ = false;
 
   // Map size for some sample keyboard layouts:
   // US: 476, French: 602, Persian: 482, Vietnamese: 1436
@@ -385,6 +392,11 @@ void PlatformKeyMap::UpdateLayout(HKL layout) {
           printable_keycode_to_key_[std::make_pair(static_cast<int>(key_code),
                                                    flags)] =
               DomKey::FromCharacter(translated_chars[0]);
+
+          // Detect whether the layout makes use of AltGraph.
+          if (flags & EF_ALTGR_DOWN) {
+            has_alt_graph_ = true;
+          }
         } else {
           // Ignores legacy non-printable control characters.
         }