[Downloads] Test both download resumption code paths.

Download resumption can take two different code paths depending on
whether the WebContents that initiated the download is still around when
the download is resumed or not. Make our tests go through both code
paths based on a parameterized test.

R=svaldez@chromium.org
BUG=7648

Review URL: https://codereview.chromium.org/1525753002

Cr-Commit-Position: refs/heads/master@{#365153}
1 file changed