[builtins] Introduce proper Float64Log1p operator.

src/base/ieee754.h

Index: src/base/ieee754.h
diff --git a/src/base/ieee754.h b/src/base/ieee754.h
index 3622320b46964bdf611a477afd39e4e709432d00..18540a51f29e7d35a93a6b1a19cef28c67230c81 100644
--- a/src/base/ieee754.h
+++ b/src/base/ieee754.h
@@ -12,6 +12,10 @@ namespace ieee754 {
 // Returns the natural logarithm of |x|.
 double log(double x);
 
+// Returns a value equivalent to |log(1+x)|, but computed in a way that is
+// accurate even if the value of |x| is near zero.
+double log1p(double x);
+
 }  // namespace ieee754
 }  // namespace base
 }  // namespace v8