Support user ownership of files in MkvWriter

Overloading MkvWriter's constructor so that it can wrap a user owned
FILE pointer. Doing this so that applications which handle the file opening
and closing by themselves don't have to have their own implementation of the
IMkvWriter interface.

Change-Id: I69c57f3e6927ea13b616c425e3b5f179d2e69215

mkvwriter.cpp

@@ -16,7 +16,10 @@
 
 namespace mkvmuxer {
 
-MkvWriter::MkvWriter() : file_(NULL) {
+MkvWriter::MkvWriter() : file_(NULL), writer_owns_file_(true) {
+}
+
+MkvWriter::MkvWriter(FILE* fp): file_(fp), writer_owns_file_(false) {
 }
 
 MkvWriter::~MkvWriter() {
@@ -56,10 +59,10 @@ bool MkvWriter::Open(const char* filename) {
 }
 
 void MkvWriter::Close() {
-  if (file_) {
+  if (file_ && writer_owns_file_) {
     fclose(file_);
-    file_ = NULL;
   }
+  file_ = NULL;
 }
 
 int64 MkvWriter::Position() const {

mkvwriter.hpp

@@ -20,6 +20,7 @@ namespace mkvmuxer {
 class MkvWriter : public IMkvWriter {
  public:
   MkvWriter();
+  MkvWriter(FILE* fp);
   virtual ~MkvWriter();
 
   // IMkvWriter interface
@@ -40,6 +41,7 @@ class MkvWriter : public IMkvWriter {
  private:
   // File handle to output file.
   FILE* file_;
+  bool writer_owns_file_;
 
   LIBWEBM_DISALLOW_COPY_AND_ASSIGN(MkvWriter);
 };