Renable warning 4701 for GN (Windows).

Partially reverts https://codereview.chromium.org/1885093002

R=tsepez@chromium.org
TBR=tsepez@chromium.org, weili@chromium.org

Review URL: https://codereview.chromium.org/1893743002 .
diff --git a/BUILD.gn b/BUILD.gn
index e27ce22..3fb5374 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -42,7 +42,10 @@
   }
 
   if (is_win) {
-    cflags += [ "/wd4267" ]
+    cflags += [
+      "/wd4267",
+      "/wd4701",
+    ]
   }
 
   if (is_clang || is_posix) {