Remove IPC::Channel::OutputElement in IPC::Channel

Nobody has used IPC::Channel::OutputElement since IPC::ChannelPosix and IPC::ChannelWin were deleted.
All callers were deleted by issue 2473993003. So this CL removes IPC::Channel::OutputElement because
it's not used anymore.

Bug: none
Change-Id: I7ff1d5abf0749e28c9ce14663110901458ede175
Reviewed-on: https://chromium-review.googlesource.com/659260
Reviewed-by: Oliver Chang <ochang@chromium.org>
Reviewed-by: Darin Fisher <darin@chromium.org>
Commit-Queue: Gyuyoung Kim <gyuyoung.kim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#502150}

ipc/ipc_channel.cc

@@ -45,14 +45,4 @@ std::string Channel::GenerateUniqueRandomChannelID() {
       base::RandInt(0, std::numeric_limits<int32_t>::max()));
 }
 
-Channel::OutputElement::OutputElement(Message* message)
-    : message_(message), buffer_(nullptr), length_(0) {}
-
-Channel::OutputElement::OutputElement(void* buffer, size_t length)
-    : message_(nullptr), buffer_(buffer), length_(length) {}
-
-Channel::OutputElement::~OutputElement() {
-  free(buffer_);
-}
-
 }  // namespace IPC

ipc/ipc_channel.h

@@ -254,26 +254,6 @@ class IPC_EXPORT Channel : public Sender {
 #endif
 
  protected:
-  // An OutputElement is a wrapper around a Message or raw buffer while it is
-  // waiting to be passed to the system's underlying IPC mechanism.
-  class OutputElement {
-   public:
-    // Takes ownership of message.
-    OutputElement(Message* message);
-    // Takes ownership of the buffer. |buffer| is freed via free(), so it
-    // must be malloced.
-    OutputElement(void* buffer, size_t length);
-    ~OutputElement();
-    size_t size() const { return message_ ? message_->size() : length_; }
-    const void* data() const { return message_ ? message_->data() : buffer_; }
-    Message* get_message() const { return message_.get(); }
-
-   private:
-    std::unique_ptr<Message> message_;
-    void* buffer_;
-    size_t length_;
-  };
-
   // Subclasses must call this method at the beginning of their implementation
   // of Connect().
   void WillConnect();