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

ppapi/cpp/dev/audio_input_dev.cc

Index: ppapi/cpp/dev/audio_input_dev.cc
diff --git a/ppapi/cpp/dev/audio_input_dev.cc b/ppapi/cpp/dev/audio_input_dev.cc
index fd8194565cf3252da6c7c0ef3cad161baf5ede59..856a41bb41692b9564081f18e5d7591861e15a47 100644
--- a/ppapi/cpp/dev/audio_input_dev.cc
+++ b/ppapi/cpp/dev/audio_input_dev.cc
@@ -31,6 +31,11 @@ AudioInput_Dev::AudioInput_Dev(Instance* instance,
   }
 }
 
+// static
+bool AudioInput_Dev::IsAvailable() {
+  return has_interface<PPB_AudioInput_Dev>();
+}
+
 bool AudioInput_Dev::StartCapture() {
   return has_interface<PPB_AudioInput_Dev>() &&
       get_interface<PPB_AudioInput_Dev>()->StartCapture(pp_resource());