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This adds a width restriction on infobars. If the webpage is wider than
the infobar max width (600dp), the infobars will "float" in the middle
bottom of the page with shadows on the left, top, and right edges of
each infobar.

BUG=396223

Review URL: https://codereview.chromium.org/1587403002

Cr-Commit-Position: refs/heads/master@{#370740}