Update from https://crrev.com/306655

net/third_party/nss/ssl/BUILD.gn

Index: net/third_party/nss/ssl/BUILD.gn
diff --git a/net/third_party/nss/ssl/BUILD.gn b/net/third_party/nss/ssl/BUILD.gn
index b8fe40fc4087d24d6e6de7749df530c3a689bf29..c958c13b9f7664fab9060048079d3749263a595c 100644
--- a/net/third_party/nss/ssl/BUILD.gn
+++ b/net/third_party/nss/ssl/BUILD.gn
@@ -8,14 +8,6 @@ config("ssl_config") {
   if (is_mac || is_win) {
     defines = [ "NSS_PLATFORM_CLIENT_AUTH" ]
   }
-
-  if (is_clang) {
-    cflags = [
-      # There is a broken header guard in /usr/include/nss/secmod.h:
-      # https://bugzilla.mozilla.org/show_bug.cgi?id=884072
-      "-Wno-header-guard",
-    ]
-  }
 }
 
 component("libssl") {
@@ -105,6 +97,11 @@ component("libssl") {
     libs = [ "Security.framework" ]
   }
 
+  if (is_clang) {
+    # SSL triggers some of these Clang warnings.
+    configs -= [ "//build/config/clang:extra_warnings" ]
+  }
+
   if (is_posix) {
     sources -= [
       "win32err.c",