Add |RTCUIApplicationStatusObserver sharedInstance| call in ios test AppDelegate.

We want to perform the observation setup as soon as possible to avoid deadlocking,
especially for test scenario where most of the work is done on main thread.

BUG=webrtc:6634

Review-Url: https://codereview.webrtc.org/3012913002
Cr-Original-Commit-Position: refs/heads/master@{#19771}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: d207a39d098259573f799aa5e6a1acff725b61a1
2 files changed