Use Dawn's BUILD.gn instead of defining another one for Skia.

Bug: chromium:1064305

Change-Id: I10ca629ff88a5492bb9f89075f6d92094340ff55
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282076
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Zhenyao Mo <zmo@google.com>
4 files changed