Web browser window size is not set to a multiple of 200x100

Originally created by @intrigeri on #6377 (Redmine)

This is https://trac.torproject.org/projects/tor/ticket/8478, partially fixed by commit 5e5c5bd41 in Torbutton 1.6.x.

We have cherry-picked this patch in Tails 0.21, but it’s not enough.

Feature Branch: feature/torbutton-1.6.11.1

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