Remove TestWebThreadBundle::REAL_TASK_SCHEDULER.

This option was also removed from content::TestBrowserThreadBundle.
https://codereview.chromium.org/2767923002

BUG=708584

Review-Url: https://codereview.chromium.org/2887463002
Cr-Commit-Position: refs/heads/master@{#471835}
2 files changed