[Downloads] Enable resumption by default (again)

Now that we are nearing the home stretch, switch to default again. It
was disabled so that we can control it via a server side experiment. But
now we are switching it back so that we get waterfall coverage.

BUG=7648

Review URL: https://codereview.chromium.org/1833153003

Cr-Commit-Position: refs/heads/master@{#383369}
1 file changed