If I check the dozens of Chrome processes on my machine, the largest part of their working sets is private, which is evidence against significant memory sharing.
It seems reasonable to only run the security sensitive parts in a dedicated process per origin and run other tasks in a central process for all web pages.
From what I understand, Chrome's processes communicate a lot via message-passing. You probably don't need large amounts of shared memory for that.
The largest part being private doesn't mean saving 10-20mb per process is insignificant. With a few electron apps open I'm easily losing a few hundred megs to electron overhead and many machines don't have that much RAM, especially laptops.
Each electron app also needs its own copy of centralized services like the i/o process, where a single chrome instance can share that across all tabs. An OS-level PWA implementation could share it across all PWAs and electron apps too.