aw: Remove global AllowWait on UI thread

content/public/browser/android/synchronous_compositor.cc

Index: content/public/browser/android/synchronous_compositor.cc
diff --git a/content/public/browser/android/synchronous_compositor.cc b/content/public/browser/android/synchronous_compositor.cc
index 2b9f39b4b7ffb6b36321752216acbb3ce762502b..f1fdfee5c78755acf7551f358ab3f3bdfd5501ed 100644
--- a/content/public/browser/android/synchronous_compositor.cc
+++ b/content/public/browser/android/synchronous_compositor.cc
@@ -6,6 +6,7 @@
 
 #include <utility>
 
+#include "base/threading/thread_restrictions.h"
 #include "cc/output/compositor_frame.h"
 
 namespace content {
@@ -36,6 +37,7 @@ SynchronousCompositor::FrameFuture::GetFrame() {
   DCHECK(!waited_);
   waited_ = true;
 #endif
+  base::ThreadRestrictions::ScopedAllowWait wait;
   waitable_event_.Wait();
   return std::move(frame_);
 }