Allow Waitable event to be used to intentionally hang the UI thread for debugging purposes.

base/threading/thread_restrictions.h

Index: base/threading/thread_restrictions.h
diff --git a/base/threading/thread_restrictions.h b/base/threading/thread_restrictions.h
index 75d89259d3dad8c24f35fe2a6f4dd7d47d7e2b3d..54f50ebca8796acf51a1f4435a8ff9071cdc2a67 100644
--- a/base/threading/thread_restrictions.h
+++ b/base/threading/thread_restrictions.h
@@ -42,6 +42,7 @@ class GpuChannelHost;
 class NestedMessagePumpAndroid;
 class RenderWidgetResizeHelper;
 class ScopedAllowWaitForAndroidLayoutTests;
+class ScopedAllowWaitForDebugURL;
 class TextInputClientMac;
 }  // namespace content
 namespace dbus {
@@ -175,6 +176,7 @@ class BASE_EXPORT ThreadRestrictions {
   friend class content::NestedMessagePumpAndroid;
   friend class content::RenderWidgetResizeHelper;
   friend class content::ScopedAllowWaitForAndroidLayoutTests;
+  friend class content::ScopedAllowWaitForDebugURL;
   friend class ::HistogramSynchronizer;
   friend class ::ScopedAllowWaitForLegacyWebViewApi;
   friend class cc::CompletionEvent;