Fix flaky AssistantOnboardingCoordinatorTest#testUseOfOutsideStrings

The CCT doesn't always have a tab by the time the Activity is resumed
(not yet with native).

Bug: 1153703
Change-Id: I1315d32b8b8ec635b865a9d661a661d3ed3f0061
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2564690Reviewed-by: Yaron Friedman <yfriedman@chromium.org>
Commit-Queue: Michael Thiessen <mthiesse@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831937}

chrome/android/features/autofill_assistant/javatests/src/org/chromium/chrome/browser/autofill_assistant/AssistantOnboardingCoordinatorTest.java

@@ -295,8 +295,10 @@ public class AssistantOnboardingCoordinatorTest {
         });
         CustomTabActivity activity = ApplicationTestUtils.waitForActivityWithClass(
                 CustomTabActivity.class, Stage.RESUMED, () -> spans[0].onClick(termsMessage));
-        CriteriaHelper.pollUiThread(
-                () -> activity.getActivityTab().getUrlString().equals(expectedTermsUrl));
+        CriteriaHelper.pollUiThread(() -> {
+            return activity.getActivityTab() != null
+                    && activity.getActivityTab().getUrlString().equals(expectedTermsUrl);
+        });
         activity.finish();
     }
 
@@ -362,7 +364,10 @@ public class AssistantOnboardingCoordinatorTest {
         String url = mActivity.getResources()
                              .getText(R.string.autofill_assistant_google_terms_url)
                              .toString();
-        CriteriaHelper.pollUiThread(() -> activity.getActivityTab().getUrlString().equals(url));
+        CriteriaHelper.pollUiThread(() -> {
+            return activity.getActivityTab() != null
+                    && activity.getActivityTab().getUrlString().equals(url);
+        });
         activity.finish();
     }