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We still see a bit of hang at buffer_manager() destruction, which is the
beginning of ContextGroup destruction. So we add a few pokes in context
destruction before the ContextGroup destruction.

BUG=749046
TEST=crash reports go down
R=piman@chromium.org

Cq-Include-Trybots: luci.chromium.try:android_optional_gpu_tests_rel;luci.chromium.try:linux_optional_gpu_tests_rel;luci.chromium.try:mac_optional_gpu_tests_rel;luci.chromium.try:win_optional_gpu_tests_rel
Change-Id: I4726e63f2cf4fa5c1471a25ef65f8d8ac9411c09
Reviewed-on: https://chromium-review.googlesource.com/1237114Reviewed-by: Antoine Labour <piman@chromium.org>
Commit-Queue: Zhenyao Mo <zmo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#592923}