impl/memory: Add "PULL" as valid taskqueue method.
impl/memory: Add "PULL" as valid taskqueue method.

BUG=

Review URL: https://codereview.chromium.org/1550863002

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