Support extracting source mapping url defined by //@ sourceMappingURL=<url> comment.

src/mirror-debugger.js

Index: src/mirror-debugger.js
diff --git a/src/mirror-debugger.js b/src/mirror-debugger.js
index 3a0353515c1d449053959fd2049d60015b3c54c7..a544be999953c9a7d9377045c2c4a5da6548009c 100644
--- a/src/mirror-debugger.js
+++ b/src/mirror-debugger.js
@@ -1782,6 +1782,11 @@ ScriptMirror.prototype.source = function() {
 };
 
 
+ScriptMirror.prototype.sourceMappingURL = function() {
+  return this.script_.sourceMappingURL();
+};
+
+
 ScriptMirror.prototype.lineOffset = function() {
   return this.script_.line_offset;
 };