Unlike Adblock Plus (the Firefox extension), Chrome Adblock is entirely useless security-wise. The reason is that Chrome Adblock doesn't really block ads, it merely hides them. Ads are still being downloaded and malicious code still gets its chance to run. It might be the same with Chrome FlashBlock, I didn't check.

Note also that process-per-tab isn't a security feature per se. As long as each process runs with user's privileges (as it is still the case for Chrome), malware doesn't need to care whether the process it gets injected into is the "one for everything" or "this tab only" process. For now it is merely a stability feature.