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There is an experimental feature in Chrome that will let you do this.
Go to chrome://flags/
Look down the list for “FPS Counter” and enable the feature.
However, there is a reason this is still considered an experimental feature. The FPS shown will actually be the average rate at which the video is being rendered by the browser, not the actual frame rate of the video. Still, this will hopefully be of some use to you.
Reddit user Sproxxx put together an illustration that explains what you will be seeing: https://i.imgur.com/QkJ2vcu.png