Fix some sign-compare warnings in webrtc/api.

webrtc/api/videocapturertracksource.cc

 /*
  *  Copyright 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
@@ skipping 296 matching lines @@
   if (formats.empty()) {
     if (video_capturer_->IsScreencast()) {
       // The screen capturer can accept any resolution and we will derive the
       // format from the constraints if any.
       // Note that this only affects tab capturing, not desktop capturing,
       // since the desktop capturer does not respect the VideoFormat passed in.
       formats.push_back(cricket::VideoFormat(kDefaultFormat));
     } 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]));
       }
     }
   }
 
   if (constraints) {
     MediaConstraintsInterface::Constraints mandatory_constraints =
         constraints->GetMandatory();
     MediaConstraintsInterface::Constraints optional_constraints;
     optional_constraints = constraints->GetOptional();
@@ skipping 72 matching lines @@
                   capture_state));
     return;
   }
 
   if (capturer == video_capturer_.get()) {
     SetState(GetReadyState(capture_state));
   }
 }
 
 }  // namespace webrtc