Pepper: Add IsAvailable() static methods to C++ wrappers to AudioInput_Dev and VideoCapture_Dev.

ppapi/cpp/dev/video_capture_dev.h

Index: ppapi/cpp/dev/video_capture_dev.h
diff --git a/ppapi/cpp/dev/video_capture_dev.h b/ppapi/cpp/dev/video_capture_dev.h
index af996ed21bd6fc50f005b47c1829b355f63a29fc..16c9d2bf5a0beec26d0386d01919fc84e45bc3e2 100644
--- a/ppapi/cpp/dev/video_capture_dev.h
+++ b/ppapi/cpp/dev/video_capture_dev.h
@@ -18,6 +18,9 @@ class VideoCapture_Dev : public Resource {
   VideoCapture_Dev(PP_Resource resource);
   VideoCapture_Dev(const VideoCapture_Dev& other);
 
+  // Returns true if the required interface is available.
+  static bool IsAvailable();
+
   int32_t StartCapture(const PP_VideoCaptureDeviceInfo_Dev& requested_info,
                        uint32_t buffer_count);
   int32_t ReuseBuffer(uint32_t buffer);