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As per crbug.com/788022 Comments 1,6:
- Consolidate 2 paragraphs into one
- Fix padding between content area and buttons
- Update "Don't show" checkbox text
- Move "Don't show" checkbox from button to content area
- Remove X (Close) button from upper right

Bug: 788022
Test: Try to open second (multi) profile, view Multiprofile warning
dialog, marvel at the unbearable Harmoniousness of it all.

Change-Id: I1bb57e06b11c66e9b3cb3f3f4149ec8edcd35af5
Reviewed-on: https://chromium-review.googlesource.com/1015786Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Commit-Queue: Greg Levin <glevin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#555453}