Shrink critical-section scope in ReceiveStatisticsImpl::GetActiveStatisticians()

The critical-section's scope can be shrunk (we can hold the lock for a shorter time).

BUG=None

Review-Url: https://codereview.webrtc.org/2984973002
Cr-Original-Commit-Position: refs/heads/master@{#19137}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 7fb11d737661f5b8c00baeab278e9f08c64a1be1
1 file changed