[iOS] Define global assert_no_deps and use it to prevent regressions.

ui/events/BUILD.gn

Index: ui/events/BUILD.gn
diff --git a/ui/events/BUILD.gn b/ui/events/BUILD.gn
index ecdf795dee818fd7f9c93eeb5e800c4739a311c1..cc5ab080c43f705d52da6bd592cc29813871c85d 100644
--- a/ui/events/BUILD.gn
+++ b/ui/events/BUILD.gn
@@ -10,6 +10,10 @@ if (is_android) {
   import("//build/config/android/rules.gni")
 }
 
+if (is_ios) {
+  import("//ios/build/config.gni")
+}
+
 static_library("dom_keycode_converter") {
   sources = [
     "keycodes/dom/dom_code.h",
@@ -459,6 +463,10 @@ test("events_unittests") {
   if (is_android) {
     sources += [ "android/motion_event_android_unittest.cc" ]
   }
+
+  if (is_ios) {
+    assert_no_deps = ios_assert_no_deps
+  }
 }
 
 if (is_android) {