Bug 1215502: Perform nsIException instance test for XPCOM exceptions r=automatedtester

The current test of the `result' property does not always pass for all
types of XPCOM exceptions.  A safer test is to do an instance check
against Components.interfaces.nsIException.

This fixes hangs such as the one described in bug 1202576.

r=dburns

--HG--
extra : rebase_source : 040d84c83ea8462c852d1bede96375d7133bf319

testing/marionette/error.js

@@ -4,7 +4,7 @@
 
 "use strict";
 
-var {results: Cr, utils: Cu} = Components;
+var {interfaces: Ci, utils: Cu} = Components;
 
 const errors = [
   "ElementNotAccessibleError",
@@ -31,32 +31,12 @@ const errors = [
 
 this.EXPORTED_SYMBOLS = ["error"].concat(errors);
 
-// Because XPCOM is a cesspool of undocumented odd behaviour,
-// Object.getPrototypeOf(err) causes another exception if err is an XPCOM
-// exception, and cannot be used to determine if err is a prototypal Error.
-//
-// Consequently we need to check for properties in its prototypal chain
-// (using in, instead of err.hasOwnProperty because that causes other
-// issues).
-//
-// Since the input is arbitrary it might _not_ be an Error, and can as
-// such be an object with a "result" property without it being considered to
-// be an exception.  The solution is to build a lookup table of XPCOM
-// exceptions from Components.results and check if the value of err#results
-// is in that table.
-const XPCOM_EXCEPTIONS = [];
-{
-  for (let prop in Cr) {
-    XPCOM_EXCEPTIONS.push(Cr[prop]);
-  }
-}
-
 this.error = {};
 
 /**
  * Checks if obj is an instance of the Error prototype in a safe manner.
  * Prefer using this over using instanceof since the Error prototype
- * isn't unique across browsers, and XPCOM exceptions are special
+ * isn't unique across browsers, and XPCOM nsIException's are special
  * snowflakes.
  *
  * @param {*} val
@@ -67,7 +47,7 @@ this.error = {};
 error.isError = function(val) {
   if (val === null || typeof val != "object") {
     return false;
-  } else if ("result" in val && val.result in XPCOM_EXCEPTIONS) {
+  } else if (val instanceof Ci.nsIException) {
     return true;
   } else {
     return Object.getPrototypeOf(val) == "Error";