CustomTabs: update benchmark tool for warmupCompleted callback.

This callback was recently added to the CustomTabs API, but the benchmark
tool was not updated and crashes when receiving this unexpected callback.

Change-Id: I4288af96791f4ac5087a8a16ad5cb9a25a150b58
Reviewed-on: https://chromium-review.googlesource.com/964323Reviewed-by: Benoit L <lizeb@chromium.org>
Commit-Queue: Matthew Cary <mattcary@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543365}

tools/android/customtabs_benchmark/java/src/org/chromium/customtabs/test/MainActivity.java

@@ -287,7 +287,10 @@ public class MainActivity extends Activity implements View.OnClickListener {
 
         @Override
         public void extraCallback(String callbackName, Bundle args) {
-            assert "NavigationMetrics".equals(callbackName);
+            if (!"NavigationMetrics".equals(callbackName)) {
+                Log.w(TAG, "Unknown extra callback skipped: " + callbackName);
+                return;
+            }
             long firstPaintMs = args.getLong("firstContentfulPaint", NONE);
             long navigationStartMs = args.getLong("navigationStart", NONE);
             if (firstPaintMs == NONE || navigationStartMs == NONE) return;