Reland [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).

patch from issue 2637853002 at patchset 20001
(http://crrev.com/2637853002#ps20001)

BUG=673253

Review-Url: https://codereview.chromium.org/2770733003
Cr-Original-Commit-Position: refs/heads/master@{#461841}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 45196266f164836fca6d41e8f9cfad0eee0b91a6

config/BUILD.gn

@@ -194,6 +194,12 @@ 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 ------------------------------------------------------------