This is to mitigate the chance of bookmarks duplication when first time
sync which is caused by first device send it local bookmarks only after
a poll interval(60s). When second device wants to do
BookmarkModelMerger::Merge(), it can't get anything from server because
first device hasn't sent any. So it commit its own bookmarks which will
cause duplication.
Note: Chrome has FCM invalidation ready so its first device will receive
notification within ~10s
Anthony Tseng
b0561e7db0