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When clicked, the help button will open a help article in a new
browser window, so that it's on top of the dialog but the user can
return to the dialog.

Bug: b:156258540, b:144861739
Change-Id: I1ff8f49e9926ed57f27f2eaca307d5180d574091
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2518442
Commit-Queue: Yicheng Li <yichengli@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#824669}