Make unload tests stable.

content/public/browser/render_widget_host.h

Index: content/public/browser/render_widget_host.h
diff --git a/content/public/browser/render_widget_host.h b/content/public/browser/render_widget_host.h
index 9ab9b0c2bda8f899f9148a03f387df5dc7ec6942..4fb43f99f835689d1c03c00a86ee59773a24f11f 100644
--- a/content/public/browser/render_widget_host.h
+++ b/content/public/browser/render_widget_host.h
@@ -218,6 +218,11 @@ class CONTENT_EXPORT RenderWidgetHost : public IPC::Sender {
   // warning too soon.
   virtual void RestartHangMonitorTimeout() = 0;
 
+  // Stops and disables hang monitor. This avoids flakiness in tests that need
+  // to observe things like beforeunload dialogs, which could fail if the
+  // timeout skips the dialog.
+  virtual void DisableHangMonitorForTesting() = 0;
+
   virtual void SetIgnoreInputEvents(bool ignore_input_events) = 0;
 
   // Called to notify the RenderWidget that it has been resized.