ScreenCapturerWinGdi randomly returns black frames in test environment. The root
cause is still unknown, so change ScreenCapturerWinGdi tests into MANUAL mode to
execute in test environment, but unblock other developers. We can eventually get
a failure ratio and more samples for debugging.
BUG=webrtc:6666, webrtc:6843
Review-Url: https://codereview.webrtc.org/2564173002
Cr-Commit-Position: refs/heads/master@{#15518}