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MD Bookmarks stamps a large amount of the same folder icon: 1 for every
node in the sidebar, and 1 for every item in the list. While iron-icon
is fast as far as Polymer elements go, stamping 50+ of them is still
expensive.

This CL switches all folder icons across the page to a plain divs that
have an SVG background. This appears to save at least 5% of page load
time on profiles with a large number of folders.

Bug: 727177
Cq-Include-Trybots: master.tryserver.chromium.linux:closure_compilation
Change-Id: I439f83ad3c3f645108d1a3d744cce2e402acbcee
Reviewed-on: https://chromium-review.googlesource.com/566264Reviewed-by: calamity <calamity@chromium.org>
Commit-Queue: Tim Sergeant <tsergeant@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487028}