Default constructor for file

BUG=webrtc:6177
R=perkj@webrtc.org, sprang@webrtc.org

Review URL: https://codereview.webrtc.org/2360303004 .

Cr-Original-Commit-Position: refs/heads/master@{#14473}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1d477ea6d5221c6654db4c0a7cebc68bb70aa080