Adds the UserInputMonitor implementation for Mac.

media/base/keyboard_event_counter.cc

+// Copyright 2013 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 "media/base/keyboard_event_counter.h"
+
+#include "base/atomicops.h"
+#include "base/logging.h"
+
+namespace media {
+
+KeyboardEventCounter::KeyboardEventCounter() : total_key_presses_(0) {}
+
+KeyboardEventCounter::~KeyboardEventCounter() {}
+
+void KeyboardEventCounter::Reset() {
+  base::AutoLock auto_lock(lock_);
+  pressed_keys_.clear();
+  total_key_presses_ = 0;
+}
+
+void KeyboardEventCounter::OnKeyboardEvent(ui::EventType event,
+                                           ui::KeyboardCode key_code) {
+  base::AutoLock auto_lock(lock_);

[Mark Mentovai, 2013/08/26 19:33:43]

Is it true that pressed_keys_ will now be maintained only from a single thread?
(The IO thread on Linux?) If so, get rid of this lock too and adjust the
thread-safety comment.

[jiayl, 2013/08/26 20:09:19]

UserInputMonitorLinux needs to be refactored to make it true. Done.

+  // Updates the pressed keys and the total count of key presses.
+  if (event == ui::ET_KEY_PRESSED) {
+    if (pressed_keys_.find(key_code) != pressed_keys_.end())
+      return;
+    pressed_keys_.insert(key_code);
+    base::subtle::NoBarrier_AtomicIncrement(
+        reinterpret_cast<base::subtle::AtomicWord*>(&total_key_presses_), 1);
+  } else {
+    DCHECK_EQ(ui::ET_KEY_RELEASED, event);
+    pressed_keys_.erase(key_code);
+  }
+}
+
+size_t KeyboardEventCounter::GetKeyPressCount() const {
+  return total_key_presses_;

[Mark Mentovai, 2013/08/26 19:33:43]

You should use NoBarrier_Load here now (even though it’ll just do the same
thing). See the comments in atomicops.h.

[jiayl, 2013/08/26 20:09:19]

Done.

+}
+
+}  // namespace media