Implement GL_CHROMIUM_lose_context. This is useful for testing context lost behaviour. BUG=angleproject:3379 Change-Id: If0e1538553b1761e313fc36ccde5138cd495200f Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1566141 Commit-Queue: Geoff Lang <geofflang@chromium.org> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>