SVGs don't render in gerrit or on git links, so switch the PNG for a
better user experience.

Update reference in code and delete second redundant reference.

Change-Id: I595cc5a5403d7b6741e22bcdb47447f3aad27bc1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2237533Reviewed-by: Nasko Oskov <nasko@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776819}