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https://codereview.chromium.org/476873002 introduced a regression where
shrinking the size of the extensions toolbar to 0 would not show the chevron
anymore.  Fix the regression, and add a test so it doesn't happen again.

BUG=407134

Review URL: https://codereview.chromium.org/492503004

Cr-Commit-Position: refs/heads/master@{#291754}