Introduce an IsMutable method on VideoFrameBuffer.

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 6f49e8aef9d0f1f387ff15cce119aca0699567ac..6ce806e35e5b1a490c301fd9c18374954f9528a2 100644
--- a/webrtc/common_video/video_frame_buffer.cc
+++ b/webrtc/common_video/video_frame_buffer.cc
@@ -89,8 +89,12 @@ const uint8_t* I420Buffer::data(PlaneType type) const {
   }
 }
 
+bool I420Buffer::IsMutable() {
+  return HasOneRef();
+}
+
 uint8_t* I420Buffer::MutableData(PlaneType type) {
-  RTC_DCHECK(HasOneRef());
+  RTC_DCHECK(IsMutable());
   return const_cast<uint8_t*>(
       static_cast<const VideoFrameBuffer*>(this)->data(type));
 }
@@ -144,6 +148,10 @@ NativeHandleBuffer::NativeHandleBuffer(void* native_handle,
   RTC_DCHECK_GT(height, 0);
 }
 
+bool NativeHandleBuffer::IsMutable() {
+  return false;
+}
+
 int NativeHandleBuffer::width() const {
   return width_;
 }
@@ -190,6 +198,11 @@ WrappedI420Buffer::~WrappedI420Buffer() {
   no_longer_used_cb_();
 }
 
+// Data owned by creator; never mutable.
+bool WrappedI420Buffer::IsMutable() {
+  return false;
+}
+
 int WrappedI420Buffer::width() const {
   return width_;
 }