Fix -Wpointer-bool-conversion warning in FunctionView

LLVM PR 204944 updated Clang's -Wpointer-bool-conversion and
-Wtautological-pointer-compare to look through lvalue references to
functions and warn when they are evaluated in a boolean context or
compared against null.
When passing a function designator/name without '&' to
webrtc::FunctionView, template argument deduction infers the type as an
lvalue reference to a function. Previously, FunctionView used a single
constructor for both function pointers and function references, which
checked 'f ? ... : nullptr'. Evaluating a function reference in this
ternary check triggered -Wpointer-bool-conversion.
This change splits the constructor into two mutually exclusive
constructors:
1. One for function pointers (using std::is_pointer), retaining the
   null check.
2. One for function references (using std::is_function), initializing
   call_ directly without the null check since function references can
   never be null.

Bug: chromium:531796076
Change-Id: Ibb798adddb47c5a110232e4caf956eba2a0743fe
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/488020
Reviewed-by: Danil Chapovalov <danilchap@webrtc.org>
Commit-Queue: Zequan Wu <zequanwu@google.com>
Reviewed-by: Denise Tell <dct@google.com>
Cr-Commit-Position: refs/heads/main@{#48172}

api/function_view.h

diff --git a/api/function_view.h b/api/function_view.h
index adbb5cd..1c25bed 100644
--- a/api/function_view.h
+++ b/api/function_view.h
@@ -73,14 +73,26 @@
   // result is an empty FunctionView.
   template <
       typename F,
-      typename std::enable_if<std::is_function<typename std::remove_pointer<
-          typename std::remove_reference<F>::type>::type>::value>::type* =
+      typename std::enable_if<
+          std::is_pointer<typename std::remove_reference<F>::type>::value &&
+          std::is_function<typename std::remove_pointer<
+              typename std::remove_reference<F>::type>::type>::value>::type* =
           nullptr>
   FunctionView(F&& f)
       : call_(f ? CallFunPtr<typename std::remove_pointer<F>::type> : nullptr) {
     f_.fun_ptr = reinterpret_cast<void (*)()>(f);
   }
 
+  // Constructor that accepts function references.
+  template <
+      typename F,
+      typename std::enable_if<std::is_function<
+          typename std::remove_reference<F>::type>::value>::type* = nullptr>
+  FunctionView(F&& f)
+      : call_(CallFunPtr<typename std::remove_reference<F>::type>) {
+    f_.fun_ptr = reinterpret_cast<void (*)()>(f);
+  }
+
   // Constructor that accepts nullptr. It creates an empty FunctionView.
   template <typename F,
             typename std::enable_if<std::is_same<