Set SameDocument flag for navigation context inside URLDidChangeWithoutDocumentChange.

This is needed for browser-initiated same-document back-forward
navigations. When the context is created it is unknown that navigation
will be a same document navigation, so the flag should be updated
later.

Bug: 853063
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: I93b4a7e2cfb9de56bd211d11298073fed28953ab
Reviewed-on: https://chromium-review.googlesource.com/1102116Reviewed-by: Danyao Wang <danyao@chromium.org>
Commit-Queue: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568082}

ios/web/web_state/ui/crw_web_controller.mm

@@ -5367,7 +5367,7 @@ registerLoadRequestForURL:(const GURL&)requestURL
       navigationContext =
           [self contextForPendingMainFrameNavigationWithURL:newURL];
     }
-    DCHECK(navigationContext->IsSameDocument());
+    navigationContext->SetIsSameDocument(true);
     _webStateImpl->OnNavigationStarted(navigationContext);
     [self didStartLoading];
     self.navigationManagerImpl->CommitPendingItem();