Fix crash when exiting Dawn samples

This patch fixes a crash (nullptr access) when closing the window of
Dawn samples by manually destroying all WGPU objects in main()
function. Without this step `wireHelper` (in SampleUtils.cpp) will
always be destroyed before any WGPU objects in each sample file, so a
nullptr access will occur when any WGPU object is destroyed as at that
time `dawnProcSetProcs(nullptr)` has already been called in
`~WireHelper()`.

Bug: None
Change-Id: If531b76a31352e315b39963265ebcf7c7b9b6064
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/183480
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>