[MD] Fix clipping of infobar child elements (particularly close button).

The bounds of the InfoBarView extend above the toolbar separator so
adding a layer to it doesn't help us mask the bar contents. Instead, add an
intermediate view that holds the bar elements.

BUG=585512

Review URL: https://codereview.chromium.org/1700383002

Cr-Commit-Position: refs/heads/master@{#376019}