Delete Filesystem::CreateFolder.

BUG=webrtc:6424

Review-Url: https://codereview.webrtc.org/2891923002
Cr-Original-Commit-Position: refs/heads/master@{#19766}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0677904e1b916594ab695a2282619cb3f207f03a
6 files changed