[ChromeDriver] Fix ExecuteGetStorageKeys

Fix the implementation of get local storage keys and get session storage
keys functions.

Bug: chromedriver:2177
Change-Id: Iac8e8155081190d92dc14a54f9c367337f2ebe86
Reviewed-on: https://chromium-review.googlesource.com/844354Reviewed-by: Jonathon Kereliuk <kereliuk@chromium.org>
Commit-Queue: John Chen <johnchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#526485}

chrome/test/chromedriver/window_commands.cc

@@ -1211,8 +1211,9 @@ Status ExecuteGetStorageKeys(const char* storage,
                              Timeout* timeout) {
   const char script[] =
       "var keys = [];"
-      "for (var key in %s) {"
-      "  keys.push(key);"
+      "var storage = %s;"
+      "for (var i = 0; i < storage.length; i++) {"
+      "  keys.push(storage.key(i));"
       "}"
       "keys";
   return web_view->EvaluateScript(