[builtins] Introduce proper Float64Log1p operator.

src/builtins.h

Index: src/builtins.h
diff --git a/src/builtins.h b/src/builtins.h
index 234803569ae48f31e907a7f4ad7b7a6c3b3f2863..5a2ab29ac20ff1ad63d6d5324ff78ccbc92fe70d 100644
--- a/src/builtins.h
+++ b/src/builtins.h
@@ -325,6 +325,7 @@ inline bool operator&(BuiltinExtraArguments lhs, BuiltinExtraArguments rhs) {
   V(MathClz32, 2)                     \
   V(MathFloor, 2)                     \
   V(MathLog, 2)                       \
+  V(MathLog1p, 2)                     \
   V(MathRound, 2)                     \
   V(MathSqrt, 2)                      \
   V(MathTrunc, 2)                     \
@@ -622,6 +623,8 @@ class Builtins {
   static void Generate_MathFloor(CodeStubAssembler* assembler);
   // ES6 section 20.2.2.20 Math.log ( x )
   static void Generate_MathLog(CodeStubAssembler* assembler);
+  // ES6 section 20.2.2.21 Math.log ( x )
+  static void Generate_MathLog1p(CodeStubAssembler* assembler);
   enum class MathMaxMinKind { kMax, kMin };
   static void Generate_MathMaxMin(MacroAssembler* masm, MathMaxMinKind kind);
   // ES6 section 20.2.2.24 Math.max ( value1, value2 , ...values )