Fix at-lease-one-tab assertion in prepareOverview

(cherry picked from commit e6244c2)

Bug: 992375
Change-Id: I67679acec8aa4be241545c08f003accba5f45df9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1750193
Reviewed-by: Theresa  <twellington@chromium.org>
Commit-Queue: Yue Zhang <yuezhanggg@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#686285}
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1750387
Reviewed-by: Krishna Govind <govind@chromium.org>
Cr-Commit-Position: refs/branch-heads/3880@{#8}
Cr-Branched-From: fd7bab0-refs/heads/master@{#685903}

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

@@ -721,8 +721,8 @@ void prepareOverview() {
             if (mModel.get(i).get(TabProperties.IS_SELECTED)) count++;
             mModel.get(i).set(TabProperties.IS_SELECTED, false);
         }
-        assert (count == 1)
-            : "There should be exactly one selected tab when calling "
+        assert (count == 1 || mModel.size() == 0)
+            : "There should be exactly one selected tab or no tabs at all when calling "
               + "TabListMediator.prepareOverview()";
     }