Before posting native key message from PluginInstanceChild, we need to check if the key combination could be a shortcut key, i.e., we need to check if Ctrl or Alt key is pressed. ModifierKeyState is a good class for checking it but it should be defined in an independent header file.
This patch separates the definition of ModifierKeyState to mozilla/widget/WinModifierKeyState.h and implement MaybeMatchShortcutKey() for PluginInstanceChild.
MozReview-Commit-ID: ZjLnJx02Ou
