Implement log10 via fdlibm port.

src/math.js

Index: src/math.js
diff --git a/src/math.js b/src/math.js
index 860b62f63e50d771625a76b0c5d411b84097085d..a8c42f47a96c6a70c9dc895f354c44d84ded0ed1 100644
--- a/src/math.js
+++ b/src/math.js
@@ -227,12 +227,6 @@ function MathAtanh(x) {
   return 0.5 * MathLog((1 + x) / (1 - x));
 }
 
-// ES6 draft 09-27-13, section 20.2.2.21.
-function MathLog10(x) {
-  return MathLog(x) * 0.434294481903251828;  // log10(x) = log(x)/log(10).
-}
-
-
 // ES6 draft 09-27-13, section 20.2.2.22.
 function MathLog2(x) {
   return MathLog(x) * 1.442695040888963407;  // log2(x) = log(x)/log(2).
@@ -369,7 +363,7 @@ function SetUpMath() {
     "asinh", MathAsinh,
     "acosh", MathAcosh,
     "atanh", MathAtanh,
-    "log10", MathLog10,
+    "log10", MathLog10,   // implemented by third_party/fdlibm
     "log2", MathLog2,
     "hypot", MathHypot,
     "fround", MathFroundJS,