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}
diff --git a/rtc_base/system/file_wrapper.cc b/rtc_base/system/file_wrapper.cc
index 5409d74..2828790 100644
--- a/rtc_base/system/file_wrapper.cc
+++ b/rtc_base/system/file_wrapper.cc
@@ -118,4 +118,10 @@
   return success;
 }
 
+FILE* FileWrapper::Release() {
+  FILE* file = file_;
+  file_ = nullptr;
+  return file;
+}
+
 }  // namespace webrtc