Add support for memory cleanup on process exit This patch adds a callback to cleanup memory on process exit. Bug: angleproject:6723 Test: Android CTS WrapperTest.testThreadCleanup Change-Id: Ia517d4c6ae280ddc1f17a3b6f77d437aaaad0678 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3294581 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Mohan Maiya <m.maiya@samsung.com>