Add a Release method for file wrapper

This CL adds a Release method for the FileWrapper class that allows it
to release the wrapped FILE* object without closing it.

Bug: b/155316201
Change-Id: If9ef4345724705dc7c66183f17bd8daadbdd00b6
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/174720
Commit-Queue: Per Åhgren <peah@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Niels Moller <nisse@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31183}

rtc_base/system/file_wrapper.cc

@@ -118,4 +118,10 @@ bool FileWrapper::Close() {
   return success;
 }
 
+FILE* FileWrapper::Release() {
+  FILE* file = file_;
+  file_ = nullptr;
+  return file;
+}
+
 }  // namespace webrtc

rtc_base/system/file_wrapper.h

@@ -66,6 +66,12 @@ class FileWrapper final {
   // Calling Close on an already closed file does nothing and returns success.
   bool Close();
 
+  // Releases and returns the wrapped file without closing it. This call passes
+  // the ownership of the file to the caller, and the wrapper is no longer
+  // responsible for closing it. Similarly the previously wrapped file is no
+  // longer available for the wrapper to use in any aspect.
+  FILE* Release();
+
   // Write any buffered data to the underlying file. Returns true on success,
   // false on write error. Note: Flushing when closing, is not required.
   bool Flush();