Suppress -Wreturn-std-move-in-c++11 for building in Skia

Bug: dawn:706
Change-Id: I882b58cf19a678f1eaa55cacc19a1ada67b55cce
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/44868
Auto-Submit: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
diff --git a/src/BUILD.gn b/src/BUILD.gn
index baaf5ae..49d04cd 100644
--- a/src/BUILD.gn
+++ b/src/BUILD.gn
@@ -27,6 +27,12 @@
     "${tint_spirv_tools_dir}/",
     "${tint_spirv_tools_dir}/include",
   ]
+
+  if (is_clang) {
+    # Suppress a warning enabled by Skia: Tint is C++14 so the C++11 defect
+    # detected by this warning has been fixed.
+    cflags = [ "-Wno-return-std-move-in-c++11" ]
+  }
 }
 
 config("tint_public_config") {