This cleans up another use of the proxy context.

Bug: angleproject:2714
Change-Id: Icba5bf76a3e9f811ee571529de16fd8162c76b3a
Reviewed-on: https://chromium-review.googlesource.com/1128928Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>