Add ability to retrieve a thread_name given a thread_id.

base/debug/trace_event_impl.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "base/debug/trace_event_impl.h"
 
 #include <algorithm>
 
 #include "base/bind.h"
 #include "base/debug/leak_annotations.h"
@@ skipping 47 matching lines @@
   "tracing categories exhausted; must increase TRACE_EVENT_MAX_CATEGORIES",
   "__metadata",
 };
 // The enabled flag is char instead of bool so that the API can be used from C.
 unsigned char g_category_enabled[TRACE_EVENT_MAX_CATEGORIES] = { 0 };
 const int g_category_already_shutdown = 0;
 const int g_category_categories_exhausted = 1;
 const int g_category_metadata = 2;
 int g_category_index = 3; // skip initial 3 categories
 
-// The most-recently captured name of the current thread
+// The name of the current thread. This is used to decide if the current
+// thread name has changed. We combine all the seen thread names into the
+// output name for the thread.
 LazyInstance<ThreadLocalPointer<const char> >::Leaky
     g_current_thread_name = LAZY_INSTANCE_INITIALIZER;
 
 }  // namespace
 
 ////////////////////////////////////////////////////////////////////////////////
 //
 // TraceEvent
 //
 ////////////////////////////////////////////////////////////////////////////////
@@ skipping 712 matching lines @@
   unsigned long long pid = static_cast<unsigned long long>(process_id_);
   process_id_hash_ = (offset_basis ^ pid) * fnv_prime;
 }
 
 void TraceLog::SetTimeOffset(TimeDelta offset) {
   time_offset_ = offset;
 }
 
 }  // namespace debug
 }  // namespace base