Fix an implicit narrowing conversion found by MSVC

Followup to https://codereview.webrtc.org/2999363002/

BUG=None

Review-Url: https://codereview.webrtc.org/3003133002
Cr-Original-Commit-Position: refs/heads/master@{#19456}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0826fb2b3594ea49238da3800056ea20f1c82c97
2 files changed