[Nav Experiment] Present native content before placeholder load.

Only for new navigations started with |loadCurrentURLInNativeView|. This
fixes the brief blank screen before NTP is presented. WebStateObserver
callbacks are not moved; they are still only triggered after placeholder
load finishes. This is because navigation manager state (e.g. last
committed item) must only be updated after WKWebView back-forward history
is updated.

Bug: 819606
Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ied642aaf0b4fdcab057faea7ee320ad5b5a88478
Reviewed-on: https://chromium-review.googlesource.com/1104301
Commit-Queue: Danyao Wang <danyao@chromium.org>
Reviewed-by: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#568266}

ios/web/navigation/navigation_item_impl.mm

@@ -119,6 +119,8 @@ const GURL& NavigationItemImpl::GetVirtualURL() const {
 }
 
 void NavigationItemImpl::SetTitle(const base::string16& title) {
+  if (title_ == title)
+    return;
   title_ = title;
   cached_display_title_.clear();
 }

ios/web/web_state/ui/crw_web_controller.mm

@@ -543,6 +543,10 @@ NSError* WKWebViewErrorWithSource(NSError* error, WKWebViewErrorSource source) {
 // Updates the WKBackForwardListItemHolder navigation item.
 - (void)updateCurrentBackForwardListItemHolder;
 
+// Presents native content using the native controller for |item| without
+// notifying WebStateObservers. This is used when loading native view for a new
+// navigation to avoid the delay introduced by placeholder navigation.
+- (void)presentNativeContentForNavigationItem:(web::NavigationItem*)item;
 // Presents native content using the native controller for |item| and notifies
 // WebStateObservers the completion of this navigation. This method does not
 // modify the underlying web view. It simply covers the web view with the native
@@ -1783,13 +1787,15 @@ registerLoadRequestForURL:(const GURL&)requestURL
     [self removeWebView];
     [self loadNativeContentForNavigationItem:self.currentNavItem];
   } else {
+    // Just present the native view now. Leave the rest of native content load
+    // until the placeholder navigation finishes.
+    [self presentNativeContentForNavigationItem:self.currentNavItem];
     [self loadPlaceholderInWebViewForURL:self.currentNavItem->GetVirtualURL()];
   }
 }
 
-- (void)loadNativeContentForNavigationItem:(web::NavigationItem*)item {
+- (void)presentNativeContentForNavigationItem:(web::NavigationItem*)item {
   const GURL targetURL = item ? item->GetURL() : GURL::EmptyGURL();
-  const web::Referrer referrer;
   id<CRWNativeContent> nativeContent =
       [_nativeProvider controllerForURL:targetURL webState:self.webState];
   // Unlike the WebView case, always create a new controller and view.
@@ -1799,6 +1805,18 @@ registerLoadRequestForURL:(const GURL&)requestURL
     item->SetVirtualURL([nativeContent virtualURL]);
   }
 
+  NSString* title = [self.nativeController title];
+  if (title && item) {
+    base::string16 newTitle = base::SysNSStringToUTF16(title);
+    item->SetTitle(newTitle);
+  }
+}
+
+- (void)loadNativeContentForNavigationItem:(web::NavigationItem*)item {
+  [self presentNativeContentForNavigationItem:item];
+
+  const GURL targetURL = item ? item->GetURL() : GURL::EmptyGURL();
+  const web::Referrer referrer;
   std::unique_ptr<web::NavigationContextImpl> navigationContext =
       [self registerLoadRequestForURL:targetURL
                              referrer:referrer
@@ -4633,7 +4651,10 @@ registerLoadRequestForURL:(const GURL&)requestURL
     webViewURL = currentWKItemURL;
   }
 
-  [self displayWebView];
+  // Don't show webview for placeholder navigation to avoid covering the native
+  // content, which may have already been shown.
+  if (!IsPlaceholderUrl(webViewURL))
+    [self displayWebView];
 
   // Record the navigation state.
   [_navigationStates setState:web::WKNavigationState::COMMITTED

ios/web/web_state/web_state_impl.mm

@@ -333,10 +333,15 @@ const base::string16& WebStateImpl::GetTitle() const {
   // match the WebContents implementation of this method.
   DCHECK(Configured());
   web::NavigationItem* item = navigation_manager_->GetLastCommittedItem();
-  if (web::GetWebClient()->IsSlimNavigationManagerEnabled() &&
-      !restored_title_.empty()) {
-    DCHECK(!item);
-    return restored_title_;
+  if (web::GetWebClient()->IsSlimNavigationManagerEnabled()) {
+    if (!restored_title_.empty()) {
+      DCHECK(!item);
+      return restored_title_;
+    }
+
+    // Display title for the visible item makes more sense. Only do this in
+    // WKBasedNavigationManager for now to limit impact.
+    item = navigation_manager_->GetVisibleItem();
   }
   return item ? item->GetTitleForDisplay() : empty_string16_;
 }