Fix flaky test ReturnToChromeTest#testInitialScrollIndex

Bug: 1023079
Change-Id: Iad81472ebfbdbd65d72391e1d9d38d23c2317a7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1907519Reviewed-by: Yue Zhang <yuezhanggg@chromium.org>
Commit-Queue: Wei-Yin Chen (陳威尹) <wychen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714318}

chrome/android/features/tab_ui/javatests/src/org/chromium/chrome/browser/tasks/tab_management/TabUiTestHelper.java

@@ -306,6 +306,12 @@ public class TabUiTestHelper {
                 () -> rule.getActivity().getTabContentManager().getPendingReadbacksForTesting()));
     }
 
+    public static void verifyAllTabsHaveThumbnail(TabModel tabModel) {
+        for (int i = 0; i < tabModel.getCount(); i++) {
+            checkThumbnailsExist(tabModel.getTabAt(i));
+        }
+    }
+
     public static void checkThumbnailsExist(Tab tab) {
         File etc1File = TabContentManager.getTabThumbnailFileEtc1(tab);
         CriteriaHelper.pollInstrumentationThread(etc1File::exists,

chrome/android/javatests/src/org/chromium/chrome/browser/tasks/ReturnToChromeTest.java

@@ -19,7 +19,6 @@ import org.junit.runner.RunWith;
 
 import org.chromium.base.test.util.CommandLineFlags;
 import org.chromium.base.test.util.Feature;
-import org.chromium.base.test.util.FlakyTest;
 import org.chromium.base.test.util.Restriction;
 import org.chromium.chrome.browser.ChromeFeatureList;
 import org.chromium.chrome.browser.ChromeSwitches;
@@ -110,10 +109,15 @@ public class ReturnToChromeTest {
     @Test
     @SmallTest
     @Feature({"ReturnToChrome", "RenderTest"})
-    @FlakyTest(message = "crbug.com/1023079")
     @CommandLineFlags.Add({BASE_PARAMS + "/" + TAB_SWITCHER_ON_RETURN_MS + "/0"})
     public void testInitialScrollIndex() throws Exception {
         TabUiTestHelper.prepareTabsWithThumbnail(mActivityTestRule, 10, 0, mUrl);
+
+        // Trigger thumbnail capturing for the last tab.
+        TabUiTestHelper.enterTabSwitcher(mActivityTestRule.getActivity());
+        TabUiTestHelper.verifyAllTabsHaveThumbnail(
+                mActivityTestRule.getActivity().getCurrentTabModel());
+
         ApplicationTestUtils.finishActivity(mActivityTestRule.getActivity());
 
         mActivityTestRule.startMainActivityFromLauncher();