BubbleFrameView: add padding if platform dislikes close buttons

On Mac, the close button is always hidden, which means that dialogs that expect
the height of the close button to be included in their insets have too little
top inset. This change causes dialogs to always include the close button size
in the top inset if the platform doesn't show close buttons.

BUG=622859

Review-Url: https://codereview.chromium.org/2189123002
Cr-Commit-Position: refs/heads/master@{#409322}