One could, but the Chromium sourcecode provided by Google is a quickly moving target. So your patch will need to be updated very often. Distro maintainers are already complaining about Chromium being a lot of work because of the rate of change and build breakage. A patch will make that worse.

Also, I would bet Google would do a few "neccessary" changes in the bookmark parts over the next releases, just to make your patch not work.

The specific problem of syncing bookmarks is probably solvable with an extension. But the move suggests that Google doesn't view Chromium as something that people will use directly, like they're not interested in people using open source Android without their proprietary components. If they gradually make more and more pieces Chrome-only, it will get harder to make a relevant browser by forking Chromium, because there will be more to reimplement before people consider using it.

And, of course, that makes perfect sense for Google. Chrome is no longer the scrappy insurgent. It's the king of the hill now, so it can start putting the fences up against competitors.