Restore disk_cache perf tests runnability.

They stopped working since the DiskCacheTestBase no longer provides
a MessageLoop as of https://codereview.chromium.org/2839663002.
(Not that I am clear as to how the disk_cache unittests get it
 created, as they don't seem to call NetTestSuite::GetScopedTaskEnvironment()

BUG=

Review-Url: https://codereview.chromium.org/2879893002
Cr-Commit-Position: refs/heads/master@{#471358}