MD Downloads: enhance comment about how "No downloads" gets populated
MD Downloads: enhance comment about how "No downloads" gets populated

Followup from https://codereview.chromium.org/1331383002/

R=groby@chromium.org
BUG=none

NOTRY=true

Review URL: https://codereview.chromium.org/1340713002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#348515}

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