Fix data race in inspector/profiler/heap-snapshot-loader.html

Make sure _reciever is closed only once to avoid exceptions in the heap snapshot worker.

BUG=330617
R=loislo@chromium.org

Review URL: https://codereview.chromium.org/131023006

git-svn-id: svn://svn.chromium.org/blink/trunk@165454 bbb929c8-8fbe-4397-9dbb-9b2b20218538