MD Downloads: show 50 downloads instead of 150
MD Downloads: show 50 downloads instead of 150

Because MD downloads is slower and we should eventually just use
<iron-list> instead to iteratively draw more items as the user scrolls.

R=groby@chromium.org
BUG=526577

NOTRY=true

Review URL: https://codereview.chromium.org/1312643008

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#348530}

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