runtime: fix race detector by recording read by chansend.
runtime: fix race detector by recording read by chansend.

R=golang-codereviews, dvyukov, khr
CC=golang-codereviews
https://codereview.appspot.com/54060043

Committer: Dmitriy Vyukov <05e8181be3150e47da8fe2e1f18d381c75496ff4@google.com>

tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go