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This change:
1) Moves the MD folder icon into //components
2) Makes the Views TreeView use it in Harmony mode

Note that there is no separate "open folder" icon in MD,
so the open and close icons are identical.

This change was previously reviewed as
<https://chromium-review.googlesource.com/c/chromium/src/+/663405>.

Bug: 610428
Change-Id: Ife816d911bef2a92497bb687c5aa59345359aa74
Reviewed-on: https://chromium-review.googlesource.com/692817
Commit-Queue: Elly Fong-Jones <ellyjones@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Reviewed-by: Trent Apted <tapted@chromium.org>
Reviewed-by: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#506822}