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This is a first step to implement new Help App dialog specs (see the bug
for details)

Bug: 1008073
Test: run on a device with a long content
Change-Id: Id1abdb069ecc52aa35590f10cb5d21ab2e7eff70
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1864674Reviewed-by: Denis Kuznetsov <antrim@chromium.org>
Commit-Queue: Roman Sorokin [CET] <rsorokin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706442}