Fix some sign-compare warnings in webrtc/api.

The disabling of the warnings doesn't seem to work when Chromium
is using our targets (https://codereview.chromium.org/2022833002)
so better fix them.

BUG=webrtc:4256,webrtc:3307
NOTRY=True

Review-Url: https://codereview.webrtc.org/2074423002
Cr-Original-Commit-Position: refs/heads/master@{#13217}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 3e33bfeb6d4a7b9e14ce856e849f5e0367689e09
5 files changed