tracing: Fix further flakiness in TracingPreferencesTest

The temp tracing file is deleted in an AsyncTask, so poll for it to
complete instead of directly asserting the file deletion.

TBR=twellington@chromium.org

Bug: 907913
Change-Id: I5c31ed9b0154756ce711bcaea0b166173a0a196f
Reviewed-on: https://chromium-review.googlesource.com/c/1349256Reviewed-by: Eric Seckler <eseckler@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610572}

chrome/android/javatests/src/org/chromium/chrome/browser/preferences/developer/TracingPreferencesTest.java

@@ -238,8 +238,8 @@ public class TracingPreferencesTest {
         deleteIntent.send();
         callbackHelper.waitForCallback(3 /* currentCallCount */);
 
-        // The temporary file should be deleted.
-        Assert.assertFalse(tempFile.exists());
+        // The temporary file should be deleted asynchronously.
+        CriteriaHelper.pollInstrumentationThread(Criteria.equals(false, () -> tempFile.exists()));
 
         // Notification should be deleted, too.
         waitForNotificationManagerMutation();