Add missing (and remove superfluous) 'explicit' from constructors.

webkit/support/web_audio_device_mock.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 WEBKIT_SUPPORT_WEB_AUDIO_DEVICE_MOCK_H_
 #define WEBKIT_SUPPORT_WEB_AUDIO_DEVICE_MOCK_H_
 
 #include "third_party/WebKit/Source/WebKit/chromium/public/platform/WebAudioDevice.h"
 
 class WebAudioDeviceMock : public WebKit::WebAudioDevice {
  public:
-  WebAudioDeviceMock(double sampleRate);
+  explicit WebAudioDeviceMock(double sampleRate);
   virtual ~WebAudioDeviceMock() { }
 
   virtual void start() { }
   virtual void stop() { }
   virtual double sampleRate();
 
  private:
   double sample_rate_;
 };
 
 #endif  // WEBKIT_SUPPORT_WEB_AUDIO_DEVICE_MOCK_H_