Fix MapAsyncAndWait use-after-free

Fixes an issue where the callback registered in
DawnTestBase::MapAsyncAndWait could cause an access violation during
global test teardown due to trying to run a callback that was allocated
on the stack and had already gone out of scope. This only occurred when
instance.WaitAny() timed out, which would cause the callback to still
be around until the instance got destroyed and ensured that all pending
callbacks were complete.

Now, the actual callback to run is provided via a shared_ptr and the
lambda provided to buffer.MapAsync() checks if it is null before running
it. After instance.WaitAny() returns but before we make any asserts, we
set the pointer to null, preventing the soon-to-be-dangling pointer from
being used in the future.

This is meant as a temporary workaround, as WaitAny() should always run
the provided callback. This can be reverted once the underlying issue
is fixed.

Bug: 459864803, 460743383
Change-Id: I5384e74ff6ab9da6104b07e6d9d235bf5d4b40c2
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/273736
Commit-Queue: Brian Sheedy <bsheedy@google.com>
Reviewed-by: Loko Kung <lokokung@google.com>
Auto-Submit: Brian Sheedy <bsheedy@google.com>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>