[Downloads] Mark download resumption as disabled by default.
[Downloads] Mark download resumption as disabled by default.

Feature state will be controlled via a server-side field trial.

BUG=7648
BUG=579108

Review URL: https://codereview.chromium.org/1589133003

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#370138}

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