[Flaky Tests] Speculative fix for BookmarkTest flakes

From flaky portal these are among top 15 flakiest:
org.chromium.chrome.browser.bookmarks.BookmarkTest#testReadingListEmptyView			
org.chromium.chrome.browser.bookmarks.BookmarkTest#testReadingListFolderShown

Sample failure looks to be because bookmarkmodel isn't loaded:
https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8857470340605361056/+/steps/chrome_public_test_apk_on_Android_device_Pixel_2__with_patch_/0/logs/Flaky_failure:_org.chromium.chrome.browser.bookmarks.BookmarkTest_testReadingListEmptyView__status_FAILURE_SUCCESS_/0

Change-Id: Ie7c050345d31fd03f3bdb4eb41b1f2253c135bd2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2643487Reviewed-by: David Trainor <dtrainor@chromium.org>
Reviewed-by: Xing Liu <xingliu@chromium.org>
Commit-Queue: Yaron Friedman <yfriedman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845901}

chrome/android/javatests/src/org/chromium/chrome/browser/bookmarks/BookmarkTest.java

@@ -1127,6 +1127,7 @@ public class BookmarkTest {
         BookmarkPromoHeader.forcePromoStateForTests(PromoState.PROMO_NONE);
         openBookmarkManager();
 
+        BookmarkTestUtil.waitForBookmarkModelLoaded();
         // Open the "Reading list" folder.
         TestThreadUtils.runOnUiThreadBlocking(
                 () -> mManager.openFolder(mBookmarkModel.getRootFolderId()));
@@ -1652,6 +1653,7 @@ public class BookmarkTest {
     public void testReadingListFolderShown() throws Exception {
         BookmarkPromoHeader.forcePromoStateForTests(PromoState.PROMO_NONE);
         openBookmarkManager();
+        BookmarkTestUtil.waitForBookmarkModelLoaded();
         TestThreadUtils.runOnUiThreadBlocking(
                 () -> mManager.openFolder(mBookmarkModel.getRootFolderId()));
         RecyclerViewTestUtils.waitForStableRecyclerView(mItemsContainer);