[iOS] Disable NSAssert and GTMDevAssert on release builds.

This was present in build/common.gypi and was inadvertently dropped
during the conversion to GN (removed by http://crrev.com/2355673003,
but hadn't been added to GN).

BUG=673253

Review-Url: https://codereview.chromium.org/2637853002
Cr-Original-Commit-Position: refs/heads/master@{#444355}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 32c54cffc1691c3b07501a86d72e75cebbf0e87f

config/BUILD.gn

@@ -221,6 +221,12 @@ config("release") {
       defines += [ "DYNAMIC_ANNOTATIONS_ENABLED=0" ]
     }
   }
+
+  if (is_ios) {
+    # Disable NSAssert and GTMDevAssert (from Google Toolbox for Mac). This
+    # follows XCode's default behavior for Release builds.
+    defines += [ "NS_BLOCK_ASSERTIONS=1" ]
+  }
 }
 
 # Default libraries ------------------------------------------------------------