Android: cronet perf tests explicitly run on SERIAL_EXECUTOR

We would like to get rid of all usages of execute() since it is unclear
whether it goes to SERIAL_EXECUTOR or THREAD_POOL_EXECUTOR (many assume
THREAD_POOL_EXECUTOR, but it actually goes to SERIAL_EXECUTOR). This is
just cleaning up this test usage.

Bug: 869907
Cq-Include-Trybots: master.tryserver.chromium.android:android_cronet_tester
Change-Id: Id783526b0daead7f413f55bb443e9d9792142667
Reviewed-on: https://chromium-review.googlesource.com/1181540
Commit-Queue: Sam Maier <smaier@chromium.org>
Commit-Queue: Paul Jensen <pauljensen@chromium.org>
Reviewed-by: Paul Jensen <pauljensen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#584487}

components/cronet/android/test/javaperftests/src/org/chromium/net/CronetPerfTestActivity.java

@@ -622,6 +622,6 @@ public class CronetPerfTestActivity extends Activity {
         PathUtils.setPrivateDataDirectorySuffix(PRIVATE_DATA_DIRECTORY_SUFFIX);
         mConfig = getIntent().getData();
         // Execute benchmarks on another thread to avoid networking on main thread.
-        new BenchmarkTask().execute();
+        new BenchmarkTask().executeOnExecutor(AsyncTask.SERIAL_EXECUTOR);
     }
 }