Android: MockSyncContentResolverDelegate explicit executeOnExecutor

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
Change-Id: I7564a0cc743f803b07aa04651337c857b7eb6d87
Reviewed-on: https://chromium-review.googlesource.com/1162704Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Commit-Queue: Sam Maier <smaier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#582249}

components/sync/test/android/javatests/src/org/chromium/components/sync/test/util/MockSyncContentResolverDelegate.java

@@ -212,7 +212,7 @@ public class MockSyncContentResolverDelegate implements SyncContentResolverDeleg
                         pendingObserverCount.release();
                     }
                 }
-                        .execute();
+                        .executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);
             } else {
                 mSyncStatusObserver.onStatusChanged(ContentResolver.SYNC_OBSERVER_TYPE_SETTINGS);
                 pendingObserverCount.release();