Bug 1150041: Fix max_len extraction in GeckoDriver#multiAction

Fixes use of pinch() routine in gestures.py as part of Gaia UI tests.
Regressed as part of bug 1107706.

r=dburns

--HG--
extra : rebase_source : a22302712be7cf2eb8ac49fa2d221b8a2ea923f8

testing/marionette/driver.js

@@ -1924,7 +1924,7 @@ GeckoDriver.prototype.multiAction = function(cmd, resp) {
     case Context.CONTENT:
       this.addFrameCloseListener("multi action chain");
       yield this.listener.multiAction(
-          {value: value, maxlen: max_len} = cmd.parameters);
+          {value: value, max_len: maxlen} = cmd.parameters);
       break;
   }
 };