If they're after you in particular, yes. But the standards for seizure at the border are basically at the customs officer's discretion and requires no forethought or preparation. They can just decide you are suspicious and take your machine, making a copy if they like.
Also, you can encrypt all settings and history locally before syncing in ChromeOS. While that won't help your gmail account or search history, those are products you can much more easily use alternatives to while still using ChromeOS itself. That leaves you vulnerable pretty much only to the authorities forcing google to send you a corrupted update, which can't be trivial and is an even higher barrier protecting you from casual and dragnet snooping.
Many not-high-profile people are forced to turn over their electronics at the border on a regular basis. A device that you can wipe and restore once you're through customs is quite valid here.
No, in this case the authorities would have asked google to seize/snoop on your account ages ago, so they wouldn't need to bother.
ChromeOS may be the right answer in some cases, but secure against authorities it is not - please don't kid yourself.