Revert of 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 6ce806e35e5b1a490c301fd9c18374954f9528a2..6f49e8aef9d0f1f387ff15cce119aca0699567ac 100644
--- a/webrtc/common_video/video_frame_buffer.cc
+++ b/webrtc/common_video/video_frame_buffer.cc
@@ -89,12 +89,8 @@
   }
 }
 
-bool I420Buffer::IsMutable() {
-  return HasOneRef();
-}
-
 uint8_t* I420Buffer::MutableData(PlaneType type) {
-  RTC_DCHECK(IsMutable());
+  RTC_DCHECK(HasOneRef());
   return const_cast<uint8_t*>(
       static_cast<const VideoFrameBuffer*>(this)->data(type));
 }
@@ -146,10 +142,6 @@
   RTC_DCHECK(native_handle != nullptr);
   RTC_DCHECK_GT(width, 0);
   RTC_DCHECK_GT(height, 0);
-}
-
-bool NativeHandleBuffer::IsMutable() {
-  return false;
 }
 
 int NativeHandleBuffer::width() const {
@@ -198,11 +190,6 @@
   no_longer_used_cb_();
 }
 
-// Data owned by creator; never mutable.
-bool WrappedI420Buffer::IsMutable() {
-  return false;
-}
-
 int WrappedI420Buffer::width() const {
   return width_;
 }