Pass the proxy context to the onDestroyDefault call.

BUG=angleproject:2464

Change-Id: I47f869d623e238ca3680c962387ba9359e7d3844
Reviewed-on: https://chromium-review.googlesource.com/1052220Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>