Fix nesting of trace events in ScopedMayLoadLibraryAtBackgroundPriority

The nested "Priority Increased" event must end before the parent event
so that it doesn't appear to outlive the parent.

R=fdoray

Change-Id: I673d7c9b364e5ac3321fabcb0a3fc6ddd5cdad88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1993802
Commit-Queue: Joe Mason <joenotcharles@chromium.org>
Reviewed-by: François Doray <fdoray@chromium.org>
Cr-Commit-Position: refs/heads/master@{#730014}

base/threading/scoped_thread_priority.cc

@@ -37,7 +37,8 @@ bool ScopedMayLoadLibraryAtBackgroundPriority::OnScopeFirstEntered() {
 
 ScopedMayLoadLibraryAtBackgroundPriority::
     ~ScopedMayLoadLibraryAtBackgroundPriority() {
-  TRACE_EVENT_END0("base", "ScopedMayLoadLibraryAtBackgroundPriority");
+  // Trace events must be closed in reverse order of opening so that they nest
+  // correctly.
 #if defined(OS_WIN)
   if (original_thread_priority_) {
     TRACE_EVENT_END0(
@@ -46,6 +47,7 @@ ScopedMayLoadLibraryAtBackgroundPriority::
     PlatformThread::SetCurrentThreadPriority(original_thread_priority_.value());
   }
 #endif  // OS_WIN
+  TRACE_EVENT_END0("base", "ScopedMayLoadLibraryAtBackgroundPriority");
 }
 
 }  // namespace internal