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1.When we add new page or new folder after selecting some page bookmarked entry
in Bookmark manager, then new item should be added immediately below the
selected item.
2.when we add new page or new folder after selecting some folder bookmarked entry
in Bookmark manager, then new item should be added as last item inside selected
folder.
3.when we add new page or new folder without selecting any bookmarked entry
in Bookmark manager, then new item should be added in the last of already existing
bookmarked entries.

Added code in addPage() and newFolder() in main.js for getting the index of new node to be added so that it will be at proper location.

BUG=409816

Review URL: https://codereview.chromium.org/795483004

Cr-Commit-Position: refs/heads/master@{#310719}