Fix AboutNewTabNavigation if slim-nav and store-pending-item-in-context are enabled.

Add missing expectation for DidChangeBackForwardState callback.

Bug: 899827
Change-Id: I1977a31ea43ceb799ec155d6d35127613003f332
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1542560
Auto-Submit: Eugene But <eugenebut@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645382}

ios/web/web_state/web_state_observer_inttest.mm

@@ -979,6 +979,9 @@ TEST_P(WebStateObserverTest, AboutNewTabNavigation) {
           &context, &nav_id));
   EXPECT_CALL(*decider_, ShouldAllowResponse(_, /*for_main_frame=*/true))
       .WillOnce(Return(true));
+  if (GetWebClient()->IsSlimNavigationManagerEnabled()) {
+    EXPECT_CALL(observer_, DidChangeBackForwardState(web_state()));
+  }
   EXPECT_CALL(observer_, DidFinishNavigation(web_state(), _))
       .WillOnce(VerifyNewPageFinishedContext(
           web_state(), second_url, kExpectedMimeType, /*content_is_html=*/true,