chrome: Add ability to handle chrome menu/keyboard commands (fixes issue #3280)

This change adds a CefCommandHandler::OnChromeCommand callback for optionally
handling Chrome commands triggered via menus or keyboard shortcuts. Supported
command IDs are listed in a new cef_command_ids.h header file.

To test: Run `cefclient --enable-chrome-runtime --hide-controls`. Most commands
will blocked and removed from context menus.

include/internal/cef_types.h

@@ -955,15 +955,66 @@ typedef enum {
 ///
 typedef enum {
   WOD_UNKNOWN,
+
+  ///
+  // Current tab. This is the default in most cases.
+  ///
   WOD_CURRENT_TAB,
+
+  ///
+  // Indicates that only one tab with the url should exist in the same window.
+  ///
   WOD_SINGLETON_TAB,
+
+  ///
+  // Shift key + Middle mouse button or meta/ctrl key while clicking.
+  ///
   WOD_NEW_FOREGROUND_TAB,
+
+  ///
+  // Middle mouse button or meta/ctrl key while clicking.
+  ///
   WOD_NEW_BACKGROUND_TAB,
+
+  ///
+  // New popup window.
+  ///
   WOD_NEW_POPUP,
+
+  ///
+  // Shift key while clicking.
+  ///
   WOD_NEW_WINDOW,
+
+  ///
+  // Alt key while clicking.
+  ///
   WOD_SAVE_TO_DISK,
+
+  ///
+  // New off-the-record (incognito) window.
+  ///
   WOD_OFF_THE_RECORD,
-  WOD_IGNORE_ACTION
+
+  ///
+  // Special case error condition from the renderer.
+  ///
+  WOD_IGNORE_ACTION,
+
+  ///
+  // Activates an existing tab containing the url, rather than navigating.
+  // This is similar to SINGLETON_TAB, but searches across all windows from
+  // the current profile and anonymity (instead of just the current one);
+  // closes the current tab on switching if the current tab was the NTP with
+  // no session history; and behaves like CURRENT_TAB instead of
+  // NEW_FOREGROUND_TAB when no existing tab is found.
+  ///
+  WOD_SWITCH_TO_TAB,
+
+  ///
+  // Creates a new document picture-in-picture window showing a child WebView.
+  ///
+  WOD_NEW_PICTURE_IN_PICTURE,
 } cef_window_open_disposition_t;
 
 ///