Replace trampolining in FFmpegDemuxer with explicit thread calling convention checks.

media/base/bind_to_loop.h.pump

 $$ This is a pump file for generating file templates.  Pump is a python
 $$ script that is part of the Google Test suite of utilities.  Description
 $$ can be found here:
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 $$ See comment for MAX_ARITY in base/bind.h.pump.
 $var MAX_ARITY = 7
 
@@ skipping 71 matching lines @@
 
 }  // namespace internal
 
 template<typename T>
 static base::Callback<T> BindToLoop(
     const scoped_refptr<base::MessageLoopProxy>& loop,
     const base::Callback<T>& cb) {
   return base::Bind(&internal::TrampolineHelper<T>::Run, loop, cb);
 }
 
+template<typename T>
+static base::Callback<T> BindToCurrentLoop(
+    const base::Callback<T>& cb) {
+  return BindToLoop(base::MessageLoopProxy::current(), cb);
+}
+
 }  // namespace media
 
 #endif  // MEDIA_BASE_BIND_TO_LOOP_H_