Re-enable Thread::Stop() sequence checks and consequently remove |thread_lock|.

base/threading/thread.cc

Index: base/threading/thread.cc
diff --git a/base/threading/thread.cc b/base/threading/thread.cc
index 98610a63c09875878ffceca6417d7af16980ca89..7dae79b4c4d2fd81b4349a642fbfa0733a4bb363 100644
--- a/base/threading/thread.cc
+++ b/base/threading/thread.cc
@@ -95,17 +95,11 @@ bool Thread::StartWithOptions(const Options& options) {
   message_loop_ = message_loop_owned.get();
   start_event_.Reset();
 
-  // Hold |thread_lock_| while starting the new thread to synchronize with
-  // Stop() while it's not guaranteed to be sequenced (until crbug/629139 is
-  // fixed).
-  {
-    AutoLock lock(thread_lock_);
-    if (!PlatformThread::CreateWithPriority(options.stack_size, this, &thread_,
-                                            options.priority)) {
-      DLOG(ERROR) << "failed to create thread";
-      message_loop_ = nullptr;
-      return false;
-    }
+  if (!PlatformThread::CreateWithPriority(options.stack_size, this, &thread_,
+                                          options.priority)) {
+    DLOG(ERROR) << "failed to create thread";
+    message_loop_ = nullptr;
+    return false;
   }
 
   // The ownership of |message_loop_| is managed by the newly created thread
@@ -135,11 +129,7 @@ bool Thread::WaitUntilThreadStarted() const {
 }
 
 void Thread::Stop() {
-  // TODO(gab): Fix improper usage of this API (http://crbug.com/629139) and
-  // enable this check, until then synchronization with Start() via
-  // |thread_lock_| is required...
-  // DCHECK(owning_sequence_checker_.CalledOnValidSequencedThread());
-  AutoLock lock(thread_lock_);
+  DCHECK(owning_sequence_checker_.CalledOnValidSequencedThread());
 
   if (thread_.is_null())
     return;
@@ -162,9 +152,7 @@ void Thread::Stop() {
 }
 
 void Thread::StopSoon() {
-  // TODO(gab): Fix improper usage of this API (http://crbug.com/629139) and
-  // enable this check.
-  // DCHECK(owning_sequence_checker_.CalledOnValidSequencedThread());
+  DCHECK(owning_sequence_checker_.CalledOnValidSequencedThread());
 
   if (stopping_ || !message_loop_)
     return;