Fix flakiness in postMessage-related SitePerProcessBrowserTests.

content/public/test/browser_test_utils.cc

Index: content/public/test/browser_test_utils.cc
diff --git a/content/public/test/browser_test_utils.cc b/content/public/test/browser_test_utils.cc
index 60423961dd6d2b7f4ad41539431a0318f92d0157..214b2fd61d678cca452e21f5e3fcde487f0bc5a8 100644
--- a/content/public/test/browser_test_utils.cc
+++ b/content/public/test/browser_test_utils.cc
@@ -75,9 +75,11 @@ class DOMOperationObserver : public NotificationObserver,
                const NotificationDetails& details) override {
     DCHECK(type == NOTIFICATION_DOM_OPERATION_RESPONSE);
     Details<DomOperationNotificationDetails> dom_op_details(details);
-    response_ = dom_op_details->json;
-    did_respond_ = true;
-    message_loop_runner_->Quit();
+    if (!did_respond_) {
+      response_ = dom_op_details->json;
+      did_respond_ = true;
+      message_loop_runner_->Quit();
+    }
   }
 
   // Overridden from WebContentsObserver: