Use base::AtomicSequenceNumber for the flush id in GLES2Implementation

Instead of writing it ourselves. Note that StaticAtomicSequenceNumber
does not work here since it can only be used in global scope.

R=piman@chromium.org
BUG=584497
CQ_INCLUDE_TRYBOTS=tryserver.chromium.win:win_optional_gpu_tests_rel;tryserver.chromium.mac:mac_optional_gpu_tests_rel

Review URL: https://codereview.chromium.org/1809753003

Cr-Commit-Position: refs/heads/master@{#381769}

gpu/command_buffer/client/gles2_implementation.cc

@@ -17,6 +17,7 @@
 #include <set>
 #include <sstream>
 #include <string>
+#include "base/atomic_sequence_num.h"
 #include "base/compiler_specific.h"
 #include "base/strings/string_split.h"
 #include "base/strings/stringprintf.h"
@@ -76,11 +77,10 @@ GLuint ToGLuint(const void* ptr) {
   return static_cast<GLuint>(reinterpret_cast<size_t>(ptr));
 }
 
+static base::StaticAtomicSequenceNumber g_flush_id;
+
 uint32_t GenerateNextFlushId() {
-  static base::subtle::Atomic32 flush_id = 0;
-  base::subtle::Atomic32 my_id =
-      base::subtle::Barrier_AtomicIncrement(&flush_id, 1);
-  return static_cast<uint32_t>(my_id);
+  return static_cast<uint32_t>(g_flush_id.GetNext());
 }
 
 }  // anonymous namespace