I find that the extension icon will be hidden sometimes, but sometimes are not .so I just Google it and find the tips.
we can simply drag and resize the extension bar, so that we can hide some or all of the icons behind the little arrows on the right side of the menu .
whether we hide some or all of the icons, we can simply press the little arrows, and all the extension icons will appear from the drop-down list, and it will work as normal .
it is very easy, isn’t it? the tips is come from:
http://www.howtogeek.com/howto/37911/google-chrome-adds-two-ways-to-hide-extension-icons/