Fix CopyOnWriteBuffer::cdata to return const pointer

webrtc/base/copyonwritebuffer.h

 /*
  *  Copyright 2016 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 71 matching lines @@
     }
     CloneDataIfReferenced(buffer_->capacity());
     return buffer_->data<T>();
   }
 
   // Get const pointer to the data. This will not create a copy of the
   // underlying data if it is shared with other buffers.
   template <typename T = uint8_t,
             typename std::enable_if<
                 internal::BufferCompat<uint8_t, T>::value>::type* = nullptr>
-  T* cdata() const {
+  const T* cdata() const {
     RTC_DCHECK(IsConsistent());
     if (!buffer_) {
       return nullptr;
     }
     return buffer_->data<T>();
   }
 
   size_t size() const {
     RTC_DCHECK(IsConsistent());
     return buffer_ ? buffer_->size() : 0;
@@ skipping 180 matching lines @@
     return (!buffer_ || buffer_->capacity() > 0);
   }
 
   // buffer_ is either null, or points to an rtc::Buffer with capacity > 0.
   scoped_refptr<RefCountedObject<Buffer>> buffer_;
 };
 
 }  // namespace rtc
 
 #endif  // WEBRTC_BASE_COPYONWRITEBUFFER_H_