Move FunctionView from AudioCodingModule to the rtc namespace

It's a very general type, and we're about to start needing it in other
places besides AudioCodingModule.

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2380463003
Cr-Commit-Position: refs/heads/master@{#14423}

webrtc/modules/audio_coding/include/audio_coding_module.h

diff --git a/webrtc/modules/audio_coding/include/audio_coding_module.h b/webrtc/modules/audio_coding/include/audio_coding_module.h
index 9f93ef5..fc8ae1e 100644
--- a/webrtc/modules/audio_coding/include/audio_coding_module.h
+++ b/webrtc/modules/audio_coding/include/audio_coding_module.h
@@ -16,6 +16,7 @@
 #include <vector>
 
 #include "webrtc/base/deprecation.h"
+#include "webrtc/base/function_view.h"
 #include "webrtc/base/optional.h"
 #include "webrtc/common_types.h"
 #include "webrtc/modules/audio_coding/codecs/audio_decoder_factory.h"
@@ -209,48 +210,18 @@
   virtual void RegisterExternalSendCodec(
       AudioEncoder* external_speech_encoder) = 0;
 
-  // Just like std::function, FunctionView will wrap any callable and hide its
-  // actual type, exposing only its signature. But unlike std::function,
-  // FunctionView doesn't own its callable---it just points to it. Thus, it's a
-  // good choice mainly as a function argument when the callable argument will
-  // not be called again once the function has returned.
-  template <typename T>
-  class FunctionView;  // Undefined.
-
-  template <typename RetT, typename... ArgT>
-  class FunctionView<RetT(ArgT...)> final {
-   public:
-    // This constructor is implicit, so that callers won't have to convert
-    // lambdas to FunctionView<Blah(Blah, Blah)> explicitly. This is safe
-    // because FunctionView is only a reference to the real callable.
-    template <typename F>
-    FunctionView(F&& f)
-        : f_(&f), call_(Call<typename std::remove_reference<F>::type>) {}
-
-    RetT operator()(ArgT... args) const {
-      return call_(f_, std::forward<ArgT>(args)...);
-    }
-
-   private:
-    template <typename F>
-    static RetT Call(void* f, ArgT... args) {
-      return (*static_cast<F*>(f))(std::forward<ArgT>(args)...);
-    }
-    void* f_;
-    RetT (*call_)(void* f, ArgT... args);
-  };
-
   // |modifier| is called exactly once with one argument: a pointer to the
   // unique_ptr that holds the current encoder (which is null if there is no
   // current encoder). For the duration of the call, |modifier| has exclusive
   // access to the unique_ptr; it may call the encoder, steal the encoder and
   // replace it with another encoder or with nullptr, etc.
   virtual void ModifyEncoder(
-      FunctionView<void(std::unique_ptr<AudioEncoder>*)> modifier) = 0;
+      rtc::FunctionView<void(std::unique_ptr<AudioEncoder>*)> modifier) = 0;
 
   // |modifier| is called exactly once with one argument: a const pointer to the
   // current encoder (which is null if there is no current encoder).
-  virtual void QueryEncoder(FunctionView<void(AudioEncoder const*)> query) = 0;
+  virtual void QueryEncoder(
+      rtc::FunctionView<void(AudioEncoder const*)> query) = 0;
 
   // Utility method for simply replacing the existing encoder with a new one.
   void SetEncoder(std::unique_ptr<AudioEncoder> new_encoder) {
@@ -529,7 +500,7 @@
   // the decoder being registered is iSAC.
   virtual int RegisterReceiveCodec(
       const CodecInst& receive_codec,
-      FunctionView<std::unique_ptr<AudioDecoder>()> isac_factory) = 0;
+      rtc::FunctionView<std::unique_ptr<AudioDecoder>()> isac_factory) = 0;
 
   // Registers an external decoder. The name is only used to provide information
   // back to the caller about the decoder. Hence, the name is arbitrary, and may