Reset thread context during threadCleanup Once thread cleanup is complete reset its mContext member to nullptr. Running CTS tests in batches sometimes exposes a bug where Thread::mContext is accessed after Display::threadCleanup is complete. Bug: angleproject:6723 Bug: angleproject:6798 Change-Id: Ie252954e60b5d4565697095939fd19d002dfccf3 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4072208 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>