Added SDL_IsChromebook() to determine if we're running on a Chromebook.

include/SDL_system.h

@@ -125,6 +125,11 @@ extern DECLSPEC void * SDLCALL SDL_AndroidGetActivity(void);
  */
 extern DECLSPEC SDL_bool SDLCALL SDL_IsAndroidTV(void);
 
+/**
+   \brief Return true if the application is running on a Chromebook
+ */
+extern DECLSPEC SDL_bool SDLCALL SDL_IsChromebook(void);
+
 /**
    See the official Android developer guide for more information:
    http://developer.android.com/guide/topics/data/data-storage.html