Add vptr_ignorelist.txt required for is_ubsan=true GN builds

Since the following change, is_ubsan_vptr is now enabled when is_ubsan
is enabled: https://chromium-review.googlesource.com/c/chromium/src/+/5971954
With is_ubsan_vptr enabled, the GN build expects to find an ignore list
file.

Change-Id: Ib191dfd1d4535173b1619957131f16a3670d1ffc
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/219494
Auto-Submit: Antonio Maiorano <[email protected]>
Reviewed-by: Kai Ninomiya <[email protected]>
Commit-Queue: Kai Ninomiya <[email protected]>