Fix Dawn's proxy groups to use public_deps instead of deps

To do perfect forwarding of GN targets, the proper method is to make a
group with public_deps. For example in the following:

 - Config C
 - Target T with a public_deps including C
 - Group G that proxies B
 - Target T2 that depends on G

If G uses deps = [ T ] then T2 won't see C, whereas if G uses
public_deps = [ T ] then T2 will dep on T, which will make it dep on C.

Bug: chromium:1064305
Change-Id: Iae236150c119b1a4003b957dcacf42e7759a936c
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/18965
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Austin Eng <enga@chromium.org>