Enable -Wextra-semi in mac release builds that have dcheck_always_on set.

Bug: 926235
Change-Id: I32c7a9f15f1f544caaceef28b0d34ebd643aa35d
Reviewed-on: https://chromium-review.googlesource.com/c/1486880Reviewed-by: Mark Mentovai <mark@chromium.org>
Commit-Queue: Mark Mentovai <mark@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Auto-Submit: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#635214}

build/config/compiler/BUILD.gn

@@ -1590,8 +1590,9 @@ config("chromium_code") {
 
     # TODO(thakis): Enable this for more platforms, https://crbug.com/926235
     has_dchecks = is_debug || dcheck_always_on
-    if (has_dchecks && is_linux && target_os != "chromeos" && !is_chromecast &&
-        !use_libfuzzer) {
+    if (has_dchecks &&
+        ((is_linux && target_os != "chromeos" && !is_chromecast &&
+          !use_libfuzzer) || (is_mac && !is_debug))) {
       cflags += [ "-Wextra-semi" ]
     }
   }