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Also modifies existing test to verify that the number of extension
buttons inside the menu matches the number of added extensions.

Bug: chromium:943702
Change-Id: Ifafc8cef9d3d6b110a7f4a510fa35531611329e8
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538863Reviewed-by: Collin Baker <collinbaker@chromium.org>
Commit-Queue: Peter Boström <pbos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644414}