Show a warning on the inspector page if canvas measureString returns 0 #777

See discussion at #367 (comment)

The reason the inspector page is messed up in Brave (which otherwise looks like Chrome) is that it returns 0 from measureString as an anti-fingerprint mechanism. This is trivial to detect, so we should show a warning at the top of the page in this case telling the user that the trees may not render correctly because of this (it might not be Brave, it could be privacy extensions in Chrome too, but probably users with these would know what they're running if we give them an idea of the cause).