Move thread_ conditional back under defines.

Unbreaks Windows builds.

BUG=
TBR=tommi@webrtc.org

Review URL: https://codereview.webrtc.org/1476543002 .

Cr-Original-Commit-Position: refs/heads/master@{#10778}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7d842d660ef5e053da0305bc1d9405fb070b05dc
1 file changed