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Add icons as SVG files, and update the css to use the new icons.

chrome.svg: monochrome Chrome logo
photos.svg: Google Photo's logo
upload.svg: upload arrow
wallpapers.svg: Chrome wallpapers logo

Bug: 851336
Cq-Include-Trybots: luci.chromium.try:closure_compilation
Change-Id: Ie4e724144c0889b70cbed1905e9ca86cd1c88d86
Reviewed-on: https://chromium-review.googlesource.com/1109598Reviewed-by: Michael Giuffrida <michaelpg@chromium.org>
Reviewed-by: Marc Treib <treib@chromium.org>
Commit-Queue: Kyle Milka <kmilka@chromium.org>
Cr-Commit-Position: refs/heads/master@{#569639}