[ios] Fixes a crash in Bookmarks.

If the bookmarks UI was displayed before the BookmarkModel was done
loading, we would incorrectly cache a pointer to a BookmarkNode that had
been destroyed.  Instead, do not set the root node of the
BookmarkHomeViewController until after the model has finished loading.

BUG=839883
TEST=See repro steps in bug. Displaying the Bookmarks UI immediately upon startup should not crash.

Cq-Include-Trybots: master.tryserver.chromium.mac:ios-simulator-cronet;master.tryserver.chromium.mac:ios-simulator-full-configs
Change-Id: I062171c6d588da6be666b92e0c961abb90f1af17
Reviewed-on: https://chromium-review.googlesource.com/1044353
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#556101}

ios/chrome/browser/ui/bookmarks/bookmark_home_view_controller.mm

@@ -584,6 +584,9 @@ std::vector<GURL> GetUrlsToOpen(const std::vector<const BookmarkNode*>& nodes) {
   if (![self isViewLoaded])
     return;
 
+  DCHECK(!_rootNode);
+  [self setRootNode:self.bookmarks->root_node()];
+
   int64_t unusedFolderId;
   double unusedScrollPosition;
   // Bookmark Model is loaded after presenting Bookmarks,  we need to check

ios/chrome/browser/ui/bookmarks/bookmark_interaction_controller.mm

@@ -175,7 +175,13 @@ using bookmarks::BookmarkNode;
                                               dispatcher:self.dispatcher];
   self.bookmarkBrowser.homeDelegate = self;
 
-  [self.bookmarkBrowser setRootNode:self.bookmarkModel->root_node()];
+  // Set the root node if the model has been loaded. If the model has not been
+  // loaded yet, the root node will be set in BookmarkHomeViewController after
+  // the model is finished loading.
+  if (self.bookmarkModel->loaded()) {
+    [self.bookmarkBrowser setRootNode:self.bookmarkModel->root_node()];
+  }
+
   int64_t unusedFolderId;
   double unusedScrollPosition;
   // If cache is present then reconstruct the last visited bookmark from