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.

libcef/browser/chrome/chrome_browser_delegate.h

@@ -52,6 +52,8 @@ class ChromeBrowserDelegate : public cef::BrowserDelegate {
   void SetAsDelegate(content::WebContents* web_contents,
                      bool set_delegate) override;
   bool ShowStatusBubble(bool show_by_default) override;
+  bool HandleCommand(int command_id,
+                     WindowOpenDisposition disposition) override;
 
   // WebContentsDelegate methods:
   void WebContentsCreated(content::WebContents* source_contents,