Fix CopyOnWriteBuffer::cdata to return const pointer

webrtc/base/copyonwritebuffer.h

Index: webrtc/base/copyonwritebuffer.h
diff --git a/webrtc/base/copyonwritebuffer.h b/webrtc/base/copyonwritebuffer.h
index a7e52beea53c3ce442ac3b97a1db013590e08861..8f90ee093f115f8e1b757ad599892e48d7572efe 100644
--- a/webrtc/base/copyonwritebuffer.h
+++ b/webrtc/base/copyonwritebuffer.h
@@ -89,7 +89,7 @@ class CopyOnWriteBuffer {
   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;