Fix some sign-compare warnings in webrtc/api.

webrtc/api/videocapturertracksource.cc

Index: webrtc/api/videocapturertracksource.cc
diff --git a/webrtc/api/videocapturertracksource.cc b/webrtc/api/videocapturertracksource.cc
index 2321d83635d49e3f51450ad50d93b4d43a4036e7..2bef1431de2c28b189ee97e8b40a74af3c270447 100644
--- a/webrtc/api/videocapturertracksource.cc
+++ b/webrtc/api/videocapturertracksource.cc
@@ -314,7 +314,7 @@ void VideoCapturerTrackSource::Initialize(
     } else {
       // The VideoCapturer implementation doesn't support capability
       // enumeration. We need to guess what the camera supports.
-      for (int i = 0; i < arraysize(kVideoFormats); ++i) {
+      for (uint32_t i = 0; i < arraysize(kVideoFormats); ++i) {
         formats.push_back(cricket::VideoFormat(kVideoFormats[i]));
       }
     }