[v8][wasm] Enable testing of trap handlers on macOS

WebAssembly trap handlers are about to be enabled in a field trial, see
http://cl/222217200.

R=jwd@chromium.org

Bug: chromium:906565
Change-Id: Ibf9ffe707cc16c5d76f004f75f397627be6fcbb4
Reviewed-on: https://chromium-review.googlesource.com/c/1346089Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610827}

testing/variations/fieldtrial_testing_config.json

@@ -5166,7 +5166,8 @@
         {
             "platforms": [
                 "linux",
-                "windows"
+                "windows",
+                "mac"
             ],
             "experiments": [
                 {