Add a unit test for "ALREADY_SAVED" being converted to "SUCCESS"

We recently made a change to treat the MHTML page already being saved to
success, and we added a unit test for the background loader offliner,
but didn't add one for the prerendering offliner.  This adds the test
for the prerendering offliner.

BUG=715152

Review-Url: https://codereview.chromium.org/2850953002
Cr-Commit-Position: refs/heads/master@{#468181}
1 file changed