GN/Win: warnings and warn-as-error

CQ_INCLUDE_TRYBOTS=master.client.skia.compile:Build-Win-MSVC-x86-Debug-Exceptions-Trybot,Build-Win-MSVC-x86_64-Debug-GN-Trybot,Build-Win-MSVC-x86_64-Release-GN-Trybot

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=3258

Change-Id: Ia2b85904bed1e6ca72c68abaecf6c2854795342c
Reviewed-on: https://skia-review.googlesource.com/3258
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/third_party/third_party.gni b/third_party/third_party.gni
index dd9f057..5764c97 100644
--- a/third_party/third_party.gni
+++ b/third_party/third_party.gni
@@ -24,5 +24,5 @@
 }
 
 set_defaults("third_party") {
-  configs = default_configs
+  configs = default_configs - [ "//gn:warnings" ]
 }