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Native Mac NSAlert dialogs show the dialog buttons on the TouchBar, we
want similar behaviour for Chrome dialogs.

If a Widget's delegate implements views::DialogDelegate, add buttons to
the touchbar for the "OK" and "Cancel" buttons. Note the labels might
not necessarily be labeled "OK" and "Cancel".

Nearly all dialogs and bubbles work correctly with the TouchBar this
way. One exception is the Add Bookmark bubble -
LocationBarBubbleDelegateView::GetDialogButtons() returns
ui::DIALOG_BUTTON_NONE. That makes sense for the ZoomBubble.
SaveCardBubbleViews overrides again to return both buttons and works.
The translate bubble has "TODO: This should be using GetDialogButtons."

Screenshot: http://crbug.com/660126#c13

BUG=661581

Review-Url: https://codereview.chromium.org/2469213002
Cr-Commit-Position: refs/heads/master@{#430209}