MD Downloads: move maximum number of items back to 150
MD Downloads: move maximum number of items back to 150

This is for an apples-to-apples comparison against pre-Polymer downloads
when it comes to performance.

R=esprehn@chromium.org
BUG=526583,526577

Review URL: https://codereview.chromium.org/1410713002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#354461}

axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk,axinging/chromium-crosswalk