[prpc] Avoid strings.ToValidUTF8 because it doesn't exist in go1.11.
[prpc] Avoid strings.ToValidUTF8 because it doesn't exist in go1.11.

We still use go1.11 when deploying to GAE.

R=ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org, tandrii@chromium.org

Change-Id: I1f82d250cf8893c1bbf1ee583f0e68d230a6794c
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/1991937
Reviewed-by: Nodir Turakulov <ef4933a197ef7b4b3f55f1bec4942aead3637a2a@chromium.org>
Commit-Queue: Vadim Shtayura <9f116ddb1b24f6fc1916a676eb17161b6c07dfc1@chromium.org>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go