[ObjC ARC] Converts ios/chrome/browser/web:eg_tests to ARC.

ios/chrome/browser/web/window_open_by_dom_egtest.mm

Index: ios/chrome/browser/web/window_open_by_dom_egtest.mm
diff --git a/ios/chrome/browser/web/window_open_by_dom_egtest.mm b/ios/chrome/browser/web/window_open_by_dom_egtest.mm
index 7938f833132bd52b361dd648fe7d0d668e582be0..2ffe641f5222c5319bc93e155afbb9d75a6baa17 100644
--- a/ios/chrome/browser/web/window_open_by_dom_egtest.mm
+++ b/ios/chrome/browser/web/window_open_by_dom_egtest.mm
@@ -16,6 +16,10 @@
 #import "ios/web/public/test/http_server.h"
 #include "ios/web/public/test/http_server_util.h"
 
+#if !defined(__has_feature) || !__has_feature(objc_arc)
+#error "This file requires ARC support."
+#endif
+
 using chrome_test_util::AssertMainTabCount;
 using chrome_test_util::OmniboxText;
 using chrome_test_util::TapWebViewElementWithId;
@@ -65,7 +69,7 @@ const char kTestURL[] =
 - (void)testLinkWithBlankTargetSessionStorage {
   using chrome_test_util::ExecuteJavaScript;
 
-  NSError* error = nil;
+  __unsafe_unretained NSError* error = nil;
   ExecuteJavaScript(@"sessionStorage.setItem('key', 'value');", &error);
   GREYAssert(!error, @"Error during script execution: %@", error);