cmd/heapview: dowgrade to customelements v0

I've had too many issues with customelements v1. The interface for v0
isn't as nice, and v0 will eventually be deprecated. But v1's polyfill
library is better supported, and it will be easy to upgrade later.

Change-Id: I87b376376b750167a0464c6c249930edecbd59db
Reviewed-on: https://go-review.googlesource.com/25545
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>
2 files changed