[release-branch.go1.8] godoc: add GoogleCN property to pages

Use that property to determine whether to show share functionality
or link to sites that are blocked in mainland China.

This change requires https://go-review.googlesource.com/c/52872

Change-Id: I47327f9dbd2624206564fa99eb1cc6a10b4f46db
Reviewed-on: https://go-review.googlesource.com/52873
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-on: https://go-review.googlesource.com/81075
Reviewed-by: Russ Cox <rsc@golang.org>
8 files changed