Use sequenced workers for the getting the system's font list.

content/common/font_list.h

 // Copyright (c) 2011 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.
 
 #ifndef CONTENT_COMMON_FONT_LIST_H_
 #define CONTENT_COMMON_FONT_LIST_H_
 
 #include "base/memory/scoped_ptr.h"
 
 namespace base {
 class ListValue;
 }
 
 namespace content {
 
+// Getting the list of fonts on the system is non-threadsafe on Linux for
+// versions of Pango predating 2013. This sequence token can be used to enforce
+// serial execution of get font list tasks.
+extern const char kFontListSequenceToken[];
+
 // Retrieves the fonts available on the current platform and returns them.
 // The caller will own the returned pointer. Each entry will be a list of
 // two strings, the first being the font family, and the second being the
 // localized name.
 //
 // This function is potentially slow (the system may do a bunch of I/O) so be
 // sure not to call this on a time-critical thread like the UI or I/O threads.
 //
+// Since getting the fonts is not threadsafe on Linux, use
+// |kFontListSequenceToken| to prevent race conditions.
+//
 // Most callers will want to use the GetFontListAsync function in
 // content/browser/font_list_async.h which does an asynchronous call.
 scoped_ptr<base::ListValue> GetFontList_SlowBlocking();
 
 }  // namespace content
 
 #endif  // CONTENT_COMMON_FONT_LIST_H_