Vulkan: Re-enable async pipeline cache sync by default This is a reland of c7414946b84410bfcc8705759a902577c13d9fc4 The change here is that the program, shader and blob caches are now reset _after_ the backend is torn down. Otherwise the pipeline cache sync thread's use of the caches races with that. Bug: b/246683126 Change-Id: Ifc3d67279de50a2ee2866476aa5df85d4c10d30d Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4111972 Reviewed-by: Charlie Lao <cclao@google.com> Auto-Submit: Shahbaz Youssefi <syoussefi@chromium.org> Reviewed-by: Yiwei Zhang <zzyiwei@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>