[ObjC ARC] Converts ios/web:ios_web_web_state_unittests to ARC.

ios/web/web_state/navigation_context_impl_unittest.mm

Index: ios/web/web_state/navigation_context_impl_unittest.mm
diff --git a/ios/web/web_state/navigation_context_impl_unittest.mm b/ios/web/web_state/navigation_context_impl_unittest.mm
index 7358b8ff287c8b31ad4725db19c55d290911d545..533524d469f91a8f05e38567754c525a48aaa9f7 100644
--- a/ios/web/web_state/navigation_context_impl_unittest.mm
+++ b/ios/web/web_state/navigation_context_impl_unittest.mm
@@ -9,6 +9,10 @@
 #include "testing/gtest/include/gtest/gtest.h"
 #include "testing/platform_test.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 namespace web {
 namespace {
 const char kRawResponseHeaders[] =
@@ -74,7 +78,7 @@ TEST_F(NavigationContextImplTest, Setters) {
   EXPECT_NE(response_headers_.get(), context->GetResponseHeaders());
 
   // SetErrorPage
-  NSError* error = [[[NSError alloc] init] autorelease];
+  NSError* error = [[NSError alloc] init];
   context->SetError(error);
   EXPECT_TRUE(context->IsSameDocument());
   ASSERT_TRUE(context->IsPost());