Reland Use CopyOnWriteBuffer instead of Buffer to avoid unnecessary copies.

webrtc/base/copyonwritebuffer.h

Index: webrtc/base/copyonwritebuffer.h
diff --git a/webrtc/base/copyonwritebuffer.h b/webrtc/base/copyonwritebuffer.h
index 17f2710f3ecbd404646268d4af9a003d7c2e5465..87f24bf51d6db78aecc44442c810d0d0887d8dec 100644
--- a/webrtc/base/copyonwritebuffer.h
+++ b/webrtc/base/copyonwritebuffer.h
@@ -42,7 +42,9 @@ class CopyOnWriteBuffer {
   template <typename T, typename internal::ByteType<T>::t = 0>
   CopyOnWriteBuffer(const T* data, size_t size, size_t capacity)
       : CopyOnWriteBuffer(size, capacity) {
-    std::memcpy(buffer_->data(), data, size);
+    if (buffer_) {
+      std::memcpy(buffer_->data(), data, size);
+    }
   }
 
   // Construct a buffer from the contents of an array.
@@ -123,13 +125,24 @@ class CopyOnWriteBuffer {
     return !(*this == buf);
   }
 
+  uint8_t& operator[](size_t index) {
+    RTC_DCHECK_LT(index, size());
+    return data()[index];
+  }
+
+  uint8_t operator[](size_t index) const {
+    RTC_DCHECK_LT(index, size());
+    return cdata()[index];
+  }
+
   // Replace the contents of the buffer. Accepts the same types as the
   // constructors.
   template <typename T, typename internal::ByteType<T>::t = 0>
   void SetData(const T* data, size_t size) {
     RTC_DCHECK(IsConsistent());
     if (!buffer_ || !buffer_->HasOneRef()) {
-      buffer_ = new RefCountedObject<Buffer>(data, size, size);
+      buffer_ = size > 0 ? new RefCountedObject<Buffer>(data, size)
+                         : nullptr;
     } else {
       buffer_->SetData(data, size);
     }