Only show a beforeunload dialog if a frame has had a user gesture since its load.

third_party/WebKit/Source/core/dom/Document.cpp

Index: third_party/WebKit/Source/core/dom/Document.cpp
diff --git a/third_party/WebKit/Source/core/dom/Document.cpp b/third_party/WebKit/Source/core/dom/Document.cpp
index 2f2611587cd266c7d2a511d7d9522b7051e41614..3e0b81300fc9e19cc75c3e1f86d26de2afb6c99c 100644
--- a/third_party/WebKit/Source/core/dom/Document.cpp
+++ b/third_party/WebKit/Source/core/dom/Document.cpp
@@ -3071,6 +3071,15 @@ bool Document::dispatchBeforeUnloadEvent(ChromeClient& chromeClient,
   if (!frame() || beforeUnloadEvent->returnValue().isNull())
     return true;
 
+  if (!frame()->hasReceivedUserGesture()) {
+    addConsoleMessage(ConsoleMessage::create(
+        JSMessageSource, ErrorMessageLevel,
+        "Blocked attempt to show a 'beforeunload' confirmation panel for a "
+        "frame that never had a user gesture since its load. "
+        "https://www.chromestatus.com/feature/5082396709879808"));
+    return true;
+  }
+
   if (didAllowNavigation) {
     addConsoleMessage(ConsoleMessage::create(
         JSMessageSource, ErrorMessageLevel,