Update Contextual Search Ranker test model.

Bug: 759161
Change-Id: I81acfa8caf45d460ebfc940f8d182300623769e1
Reviewed-on: https://chromium-review.googlesource.com/811751Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522211}

testing/variations/fieldtrial_testing_config.json

@@ -927,7 +927,7 @@
                 {
                     "name": "Expected",
                     "params": {
-                        "contextual-search-ranker-model-url": "https://www.gstatic.com/chrome/intelligence/assist/ranker/models/contextual_search/test_ranker_model_20171109_short_words.pb.bin"
+                        "contextual-search-ranker-model-url": "https://www.gstatic.com/chrome/intelligence/assist/ranker/models/contextual_search/test_ranker_model_20171109_short_words_v2.pb.bin"
                     },
                     "enable_features": [
                         "ContextualSearchMlTapSuppression",