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By the recent visual refresh on ext4 migration dialog, the header icon was
changed from chrome logo to warning icon, and the illustration was modified.

Bug: 835461
Test: Manually confirmed on Pixelbook.
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I639b7b22f21d73396f33e41af3dc8bfa084ad2de
Reviewed-on: https://chromium-review.googlesource.com/1025851Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Naoki Fukino <fukino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#553494}