Adjust new tab button path to get crisp line along top edge (Mac).
Adjust new tab button path to get crisp line along top edge (Mac).

BUG=590974

Review URL: https://codereview.chromium.org/1751153002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#378561}

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