Knowing the share context at native context creation time is required if
we want to honor the requested share group instead of virtualizing
contexts or using global share groups.

BUG=angleproject:2464

Change-Id: I1fb4d71de266b3191986b1754e73d474e49445bb
Reviewed-on: https://chromium-review.googlesource.com/1108743
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>