[Merge M49] [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: refs/heads/master@{#370138}
(cherry picked from commit 3e0b9d7839072b59ac19110e1e6666b93716ed41)

Review URL: https://codereview.chromium.org/1609333002 .

Cr-Commit-Position: refs/branch-heads/2623@{#22}
Cr-Branched-From: 92d77538a86529ca35f9220bd3cd512cbea1f086-refs/heads/master@{#369907}
1 file changed