Shortcut viewer: Minor fixes to |shortcut_message_id| logic

This fixes two issues:

- If |has_invalid_dom_key| is set and |item.shortcut_message_id| was
  missing, the old logic would incorrectly DCHECK instead of displaying
  the "no mapping" string.
- Dynamically changing the length of replacement strings does not change
  the shortcut message used. This isn't a problem with the current
  implementation, as the length of replacement strings is static, but
  may be a problem if replacement strings are dynamically chosen in the
  future.

Change-Id: Idfa355b68e29508125e2278dac83a60f7a215807
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2513307
Commit-Queue: Michael Cui <mlcui@google.com>
Reviewed-by: Tao Wu <wutao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#823833}

ash/shortcut_viewer/views/keyboard_shortcut_item_view.cc

@@ -102,14 +102,18 @@ KeyboardShortcutItemView::KeyboardShortcutItemView(
   }
 
   int shortcut_message_id;
-  if (item.shortcut_message_id) {
+  if (has_invalid_dom_key) {
+    // |shortcut_message_id| should never be used if the shortcut is not
+    // supported on the current keyboard layout.
+    shortcut_message_id = -1;
+  } else if (item.shortcut_message_id) {
     shortcut_message_id = *item.shortcut_message_id;
   } else {
     // Automatically determine the shortcut message based on the number of
-    // shortcut_key_codes.
+    // replacement strings.
     // As there are separators inserted between the modifiers, a shortcut with
-    // N modifiers has 2*N + 1 shortcut_key_codes.
-    switch (item.shortcut_key_codes.size()) {
+    // N modifiers has 2*N + 1 replacement strings.
+    switch (replacement_strings.size()) {
       case 1:
         shortcut_message_id = IDS_KSV_SHORTCUT_ONE_KEY;
         break;
@@ -124,7 +128,7 @@ KeyboardShortcutItemView::KeyboardShortcutItemView(
         break;
       default:
         NOTREACHED() << "Automatically determined shortcut has "
-                     << item.shortcut_key_codes.size() << " key codes.";
+                     << replacement_strings.size() << " replacement strings.";
     }
   }