This cleans up another usage of getProxyContext.

Bug: angleproject:2714
Change-Id: I5337e30cdd96e282e355c727bb40cd30e049ec1a
Reviewed-on: https://chromium-review.googlesource.com/1128926Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>