Okay found something after unnecessarily victimizing Google chrome for this problem.
uninstalled Google chrome. searched chrome in c: drive and deleted ever file with 'chrome' in it! couldn't delete a couple of them.
Then I thought, since this is happening only when I visit web.watsapp.com when I type it, I tried it on another computer in a different browser:
result: it redirects to download.yahoo... something from everywhere (all machines and browsers tried).
turns out, the real site is web.whatsapp.com (in my bookmark also, which is why it worked). what I was typing was a spelling mistake and putting that in any browser redirects it to that nasty thing.
P.S. - there is some difference on what is happening with this web address now vs earlier.
when I had not uninstalled google and disable several extensions, it was redirecting me to the website with screenshot i shared. however, after aggressively removing many things, now it is always redirecting to yahoo extension download page. still not cool, but there was something in the computer which was facilitating that bad web address to wrong domains.