Fix memory leak in VideoToolbox encoder.

We were leaking a fragmentation header object on each frame.

BUG=webrtc:8132

Review-Url: https://codereview.webrtc.org/3004013002
Cr-Original-Commit-Position: refs/heads/master@{#19602}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: f8084d485b87e49a68ff8e46e07bc01d375586be
4 files changed