Coverity: Fix several pass-by-values.

ppapi/thunk/ppb_video_capture_api.h

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef PPAPI_THUNK_VIDEO_CAPTURE_API_H_
 #define PPAPI_THUNK_VIDEO_CAPTURE_API_H_
 
 #include <string>
 #include <vector>
 
 #include "ppapi/c/dev/ppb_video_capture_dev.h"
 
 namespace ppapi {
 
 struct DeviceRefData;
 
 namespace thunk {
 
 class PPB_VideoCapture_API {
  public:
   virtual ~PPB_VideoCapture_API() {}
 
   virtual int32_t EnumerateDevices(PP_Resource* devices,
-                                   PP_CompletionCallback callback) = 0;
+                                   const PP_CompletionCallback& callback) = 0;
   virtual int32_t Open(const std::string& device_id,
                        const PP_VideoCaptureDeviceInfo_Dev& requested_info,
                        uint32_t buffer_count,
-                       PP_CompletionCallback callback) = 0;
+                       const PP_CompletionCallback& callback) = 0;
   virtual int32_t StartCapture() = 0;
   virtual int32_t ReuseBuffer(uint32_t buffer) = 0;
   virtual int32_t StopCapture() = 0;
   virtual void Close() = 0;
 
   // For backward compatibility.
   virtual int32_t StartCapture0_1(
       const PP_VideoCaptureDeviceInfo_Dev& requested_info,
       uint32_t buffer_count) = 0;
 
   // This function is not exposed through the C API, but returns the internal
   // data for easy proxying.
   virtual const std::vector<DeviceRefData>& GetDeviceRefData() const = 0;
 };
 
 }  // namespace thunk
 }  // namespace ppapi
 
 #endif  // PPAPI_THUNK_VIDEO_CAPTURE_API_H_