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

chrome/browser/devtools/devtools_sanity_browsertest.cc

Index: chrome/browser/devtools/devtools_sanity_browsertest.cc
diff --git a/chrome/browser/devtools/devtools_sanity_browsertest.cc b/chrome/browser/devtools/devtools_sanity_browsertest.cc
index a984c384e874482a36aa3278567527f447d9e452..8c12735b5d11258c654ee1c5d1527eced890cab4 100644
--- a/chrome/browser/devtools/devtools_sanity_browsertest.cc
+++ b/chrome/browser/devtools/devtools_sanity_browsertest.cc
@@ -386,6 +386,9 @@ class DevToolsBeforeUnloadTest: public DevToolsSanityTest {
     ASSERT_TRUE(content::ExecuteScript(web_contents->GetRenderViewHost(),
         "window.addEventListener('beforeunload',"
         "function(event) { event.returnValue = 'Foo'; });"));
+    // JavaScript onbeforeunload dialogs require a user gesture.
+    web_contents->GetMainFrame()->ExecuteJavaScriptWithUserGestureForTests(
+        base::string16());
     // Disable the hang monitor, otherwise there will be a race between the
     // beforeunload dialog and the beforeunload hang timer.
     web_contents->GetMainFrame()->DisableBeforeUnloadHangMonitorForTesting();