Move GetOutstandingBytes() call inside field trial check to avoid taking an unneccessary lock.

BUG=webrtc:7926

Review-Url: https://codereview.webrtc.org/3009673002
Cr-Original-Commit-Position: refs/heads/master@{#19540}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: a86f57e624100e8ca227fee8b726a59f3440f92f
1 file changed