[ios] Add close web state unittest

Adds single web state closing test to WebStateList unittests.

Change-Id: If129d6465d3e89047a044eaf8d3e25c8f450b1b7
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860124
Commit-Queue: edchin <edchin@chromium.org>
Reviewed-by: edchin <edchin@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: edchin <edchin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#705579}

ios/chrome/browser/web_state_list/web_state_list_unittest.mm

@@ -736,3 +736,21 @@ TEST_F(WebStateListTest, CloseAllWebStates) {
   EXPECT_TRUE(observer_.will_close_all_webstates_called());
   EXPECT_TRUE(observer_.did_close_all_webstates_called());
 }
+
+// Test closing one webstate.
+TEST_F(WebStateListTest, CloseWebState) {
+  AppendNewWebState(kURL0);
+  AppendNewWebState(kURL1);
+  AppendNewWebState(kURL2);
+
+  // Sanity check before closing WebState.
+  EXPECT_EQ(3, web_state_list_.count());
+
+  observer_.ResetStatistics();
+  web_state_list_.CloseWebStateAt(0, WebStateList::CLOSE_USER_ACTION);
+
+  EXPECT_EQ(2, web_state_list_.count());
+  EXPECT_TRUE(observer_.web_state_detached_called());
+  EXPECT_FALSE(observer_.will_close_all_webstates_called());
+  EXPECT_FALSE(observer_.did_close_all_webstates_called());
+}