That is already possible. In a browser, the area where the site is displayed, without borders, navbar, etc. is called the viewport.
Here's how you can enable a tooltip that displays both the window and the viewport size (and assign a keyboard shortcut to toggle it).
Version 1: http://take.ms/T70uU
Version 2: http://take.ms/K3ndR
(You can download version 2 from: https://chrome.google.com/webstore/detail/window-resizer-beta/pnhnbekjlbamfnnemcaolkjchjlidbib)
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