Allow passing in strings of length zero to FileWrapper::Write without closing the file.

BUG=webrtc:5253

Review URL: https://codereview.webrtc.org/1714503003

Cr-Commit-Position: refs/heads/master@{#12132}
diff --git a/webrtc/system_wrappers/source/file_impl.cc b/webrtc/system_wrappers/source/file_impl.cc
index 0ee0dea..e033c0e 100644
--- a/webrtc/system_wrappers/source/file_impl.cc
+++ b/webrtc/system_wrappers/source/file_impl.cc
@@ -243,13 +243,12 @@
   }
 
   size_t num_bytes = fwrite(buf, 1, length, id_);
-  if (num_bytes > 0) {
-    size_in_bytes_ += num_bytes;
-    return true;
+  size_in_bytes_ += num_bytes;
+  if (num_bytes != length) {
+    CloseFileImpl();
+    return false;
   }
-
-  CloseFileImpl();
-  return false;
+  return true;
 }
 
 int FileWrapperImpl::CloseFileImpl() {