How to keep Chrome bookmarks in-sync with Safari
To clarify the previous answer.

Known issues with Xmarks+Chrome+Safari

I would like to add some remarks about Xmarks. The use of Xmarks for Safari and Chrome works mostly very well, but in some case, you can encounter conflicts issues.

Context

If you enable Chrome bookmarks synchronisation and if Xmarks plugin is in automatic sync mode (default setting), some conflicts can appear:


all your bookmarks folder will appear in double.
One will be empty, the other contains the bookmarks.


It can be boring to fix that kind of issue manually... Especially when you have a lot of bookmarks. 

Hack/Solution

The only solution I have found is to:


Keep using Google Chrome synchronisation (bookmarks, plug-in and whatever you want).
Disable the automatic mode of Xmarks plugin (both on Safari and Chrome). 
Then, each time you add a bookmark into safari or chrome think to overwrite Xmarks server with "upload" (see Xmarks advanced settings) and then, download the new bookmarks via "download". (the image below is an Xmarks for Safari screenshot - You have almost the same panel in Chrome Xmarks plugin).


Conclusion

Of course, that trick doesn't make your life easier and is impossible to use when you want to use Safari and Chrome in the same time (too many upload/download to do).