Reduce scope of optimizing compiler dispatcher's output queue guard

... as suggested by the bug's reporter.

Bug: chromium:935358
Change-Id: Iab9d207e4f55a395d0496ef5d5ea0e9b6783ae66
Reviewed-on: https://chromium-review.googlesource.com/c/1488757Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Jaroslav Sevcik <jarin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59862}

src/compiler-dispatcher/optimizing-compile-dispatcher.cc

@@ -130,11 +130,14 @@ void OptimizingCompileDispatcher::CompileNext(OptimizedCompilationJob* job) {
   CompilationJob::Status status = job->ExecuteJob();
   USE(status);  // Prevent an unused-variable error.
 
-  // The function may have already been optimized by OSR.  Simply continue.
-  // Use a mutex to make sure that functions marked for install
-  // are always also queued.
-  base::MutexGuard access_output_queue_(&output_queue_mutex_);
-  output_queue_.push(job);
+  {
+    // The function may have already been optimized by OSR.  Simply continue.
+    // Use a mutex to make sure that functions marked for install
+    // are always also queued.
+    base::MutexGuard access_output_queue_(&output_queue_mutex_);
+    output_queue_.push(job);
+  }
+
   isolate_->stack_guard()->RequestInstallCode();
 }