[MD] Use strong call to action styling on solitary infobar buttons.
[MD] Use strong call to action styling on solitary infobar buttons.

BUG=596238

Review URL: https://codereview.chromium.org/1871133002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#386873}

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