tracing: Fix flakiness of TracingPreferencesTest#testSelectCategories

Fragment may not have immediately attached during activity creation, so
wait for it to attach in test.

TBR=twellington@chromium.org

Bug: 907224
Change-Id: I4fa6a9b9478b0d0e34579e5712da3822ebb6e8ec
Reviewed-on: https://chromium-review.googlesource.com/c/1345435Reviewed-by: Eric Seckler <eseckler@chromium.org>
Commit-Queue: Eric Seckler <eseckler@chromium.org>
Cr-Commit-Position: refs/heads/master@{#609967}

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

@@ -312,8 +312,8 @@ public class TracingPreferencesTest {
                         });
                     });
 
-            PreferenceFragment categoriesFragment =
-                    (PreferenceFragment) categoriesActivity.getFragmentForTest();
+            PreferenceFragment categoriesFragment = ActivityUtils.waitForFragmentToAttach(
+                    categoriesActivity, PreferenceFragment.class);
             Assert.assertEquals(TracingCategoriesPreferences.class, categoriesFragment.getClass());
 
             CheckBoxPreference sampleCategoryPref =