Workaround dependency on closure library in chromevox test utility.

This used to work because the accessibility audit library pulls in parts
of the closure libray. Since it doesn't leak its internals in a newer
version, this dependency problem was exposed. The underlying issue is that
GEN_INCLUDES in the js test are included before closureModuleDeps, so
declarations from the latter are not available when evaluating the former.

Needed for https://codereview.chromium.org/1447693002/

BUG=555753
R=aboxhall@chromium.org

Review URL: https://codereview.chromium.org/1472763002

Cr-Commit-Position: refs/heads/master@{#361142}

chrome/browser/resources/chromeos/chromevox/walkers/walker_unittest_base.js

@@ -39,7 +39,11 @@ CvoxWalkerUnitTestBase.prototype = {
    * Returns a new walker appropriate for the child test.
    * @return {!cvox.AbstractWalker} The walker instance.
    */
-  newWalker: goog.abstractMethod,
+  // Closure library is not available when this literal is evaluated, so
+  // we can't use goog.abstractMethod here.
+  newWalker: function() {
+    throw Error('newWalker not implemented.');
+  },
 
   /**
    * Makes testing much less verbose. Executes the command on the