Re-enable FullStackTest.ScreenshareSlidesVP9_2SL test.

TBR=sprang@webrtc.org
BUG=webrtc:7401

Review-Url: https://codereview.webrtc.org/2799063003
Cr-Original-Commit-Position: refs/heads/master@{#17571}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 5f4aaeb0a431d4e465f7d68b574b88147f7d101f