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* Sets dialog margins correctly.
* Sets body text context correctly.
* Replaces Grid with Box since no one likes GridLayout; this fixes
  bug 1119305 as a side effect since BoxLayout lays out StyledLabel at
  times/widths more like what it wants.
* Uses "v* = AddChildView(std::make_unique<V>(...));" idiom.
* Lays out the "checkbox" contents more like a real checkbox: with the
  border around the outside of everything.  This removes the need for a
  label vertical alignment hack and corrects the excessive spacing
  between checkbox and label.
* Makes Checkbox overrides of public methods public; this was necessary
  anyway to do the above.

Bug: 1119305
Change-Id: Ia7afa6ffeaeeab9ad407acbacaacc937f1fb842d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2377066
Commit-Queue: Peter Boström <pbos@chromium.org>
Reviewed-by: Peter Boström <pbos@chromium.org>
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Cr-Commit-Position: refs/heads/master@{#802255}