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

content/browser/frame_host/debug_urls.cc

Index: content/browser/frame_host/debug_urls.cc
diff --git a/content/browser/frame_host/debug_urls.cc b/content/browser/frame_host/debug_urls.cc
index 29915ef9fa83cab5a83ce9f508bee2f225703257..3f6769bac1fc385235637ef1d93177efb1929159 100644
--- a/content/browser/frame_host/debug_urls.cc
+++ b/content/browser/frame_host/debug_urls.cc
@@ -15,6 +15,7 @@
 #include "base/debug/profiler.h"
 #include "base/strings/utf_string_conversions.h"
 #include "base/synchronization/waitable_event.h"
+#include "base/threading/thread_restrictions.h"
 #include "cc/base/switches.h"
 #include "content/browser/gpu/gpu_process_host_ui_shim.h"
 #include "content/public/browser/browser_thread.h"
@@ -150,6 +151,11 @@ bool HandleAsanDebugURL(const GURL& url) {
 
 }  // namespace
 
+class ScopedAllowWaitForDebugURL {
+ private:
+  base::ThreadRestrictions::ScopedAllowWait wait;
+};
+
 bool HandleDebugURL(const GURL& url, ui::PageTransition transition) {
   // Ensure that the user explicitly navigated to this URL, unless
   // kEnableGpuBenchmarking is enabled by Telemetry.
@@ -177,6 +183,7 @@ bool HandleDebugURL(const GURL& url, ui::PageTransition transition) {
   }
 
   if (url == GURL(kChromeUIBrowserUIHang)) {
+    ScopedAllowWaitForDebugURL allow_wait;
     base::WaitableEvent(false, false).Wait();
     return true;
   }