Implement scoped_const_refptr template.

webrtc/common_video/video_frame_buffer.cc

Index: webrtc/common_video/video_frame_buffer.cc
diff --git a/webrtc/common_video/video_frame_buffer.cc b/webrtc/common_video/video_frame_buffer.cc
index 2875c661c8b3835f4c70ec706c12f9e66992038b..9107cc8a1bc10249ed5ff9f93be4e8de701d4673 100644
--- a/webrtc/common_video/video_frame_buffer.cc
+++ b/webrtc/common_video/video_frame_buffer.cc
@@ -200,7 +200,7 @@ rtc::scoped_refptr<VideoFrameBuffer> I420Buffer::NativeToI420Buffer() {
 }
 
 rtc::scoped_refptr<I420Buffer> I420Buffer::Copy(
-    const rtc::scoped_refptr<VideoFrameBuffer>& source) {
+    const rtc::scoped_const_refptr<VideoFrameBuffer>& source) {
   int width = source->width();
   int height = source->height();
   rtc::scoped_refptr<I420Buffer> target = I420Buffer::Create(width, height);
@@ -224,7 +224,7 @@ void I420Buffer::SetToBlack() {
 }
 
 void I420Buffer::CropAndScaleFrom(
-    const rtc::scoped_refptr<VideoFrameBuffer>& src,
+    const rtc::scoped_const_refptr<VideoFrameBuffer>& src,
     int offset_x,
     int offset_y,
     int crop_width,
@@ -261,7 +261,7 @@ void I420Buffer::CropAndScaleFrom(
 }
 
 void I420Buffer::CropAndScaleFrom(
-    const rtc::scoped_refptr<VideoFrameBuffer>& src) {
+    const rtc::scoped_const_refptr<VideoFrameBuffer>& src) {
   const int crop_width =
       std::min(src->width(), width() * src->height() / height());
   const int crop_height =
@@ -273,12 +273,13 @@ void I420Buffer::CropAndScaleFrom(
       crop_width, crop_height);
 }
 
-void I420Buffer::ScaleFrom(const rtc::scoped_refptr<VideoFrameBuffer>& src) {
+void I420Buffer::ScaleFrom(
+    const rtc::scoped_const_refptr<VideoFrameBuffer>& src) {
   CropAndScaleFrom(src, 0, 0, src->width(), src->height());
 }
 
 rtc::scoped_refptr<I420Buffer> I420Buffer::CopyKeepStride(
-    const rtc::scoped_refptr<VideoFrameBuffer>& source) {
+    const rtc::scoped_const_refptr<VideoFrameBuffer>& source) {
   int width = source->width();
   int height = source->height();
   int stride_y = source->StrideY();