Bug 1062571 - Keyboard is basically unusable with the new "alternative
 chars menu" UI.
  - Should not cause JS error when invoking isMenuTarget() when there is
    no alternative chars menu.
Merge pull request #23760 from RudyLu/keyboard/Bug1062571

Bug 1062571 - Keyboard is basically unusable with the new alternative chars menu.
r=timdream.