Make MD downloads toolbar a fixed width
Make MD downloads toolbar a fixed width

I was hoping that the contents of the toolbar would just be the same
height on every platform, but due to different fonts they'll never be
exactly the same.  The space is distributed equally because the items
are center aligned, so this is basically equivalent.

R=tommycli@chromium.org
BUG=425626

Review URL: https://codereview.chromium.org/1272153005

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#342033}

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