We don't need a lock for the output queue on the main thread

The output queue is supposed to be a lock-free anyways, we're just
temporarily abusing it by having multiple producers. For those, we need
the lock when enqueuing jobs.

BUG=none
R=yangguo@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/953883002

Cr-Commit-Position: refs/heads/master@{#26824}

src/optimizing-compiler-thread.cc

@@ -211,7 +211,6 @@ void OptimizingCompilerThread::FlushInputQueue(bool restore_function_code) {
 
 
 void OptimizingCompilerThread::FlushOutputQueue(bool restore_function_code) {
-  base::LockGuard<base::Mutex> access_output_queue_(&output_queue_mutex_);
   OptimizedCompileJob* job;
   while (output_queue_.Dequeue(&job)) {
     // OSR jobs are dealt with separately.