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1. Previously, we didn't animate the browser actions
   container when the visible count changed, because we set
   the width prior to calling animate. Fix it by animating to
   the desired width.
2. There was a bug where we would improperly hide an extension
   upon loading another, because we wouldn't account for the
   chevron width. Fix this, and add a test.

BUG=410348
BUG=410361

Review URL: https://codereview.chromium.org/539543002

Cr-Commit-Position: refs/heads/master@{#293947}